There is a need to do all kinds of entertainment and pleasant things in a person's life. It is true that eating food at home at a restaurant will reduce the cost, but if you can go out with your family once or twice a month or take them to a restaurant to eat, the good moments that will be created are worth much more in many . I never think it is right to save money by avoiding the pleasures and beautiful moments of life. But it is true that nothing in excess is good. And the one you mentioned about alcohol addiction must refrain from such things. Both economic and physical damage increase significantly in this.

 And the friend you mentioned is actually living an uncontrolled life. Let alone his investment in Bitcoin, if he continues like this, he may not be able to manage his own life properly in the future. Fund management and a controlled life are not only important for investing in Bitcoin, it works as a very important issue in the life of every common person.

The dca strategy is the most suitable strategy to use most expecially to newbis and also low income earners, if an investor buys consistently for a long time with their little discretionary income every weeks or months they can grow a good bitcoin portfolio just like this saying that little drop of water steadily can fill up a tank. The DCA strategy is good for all investor who wants to accumulate bitcoin and hodl it for long.

The honest truth about what @Moreno233 said above is that it has both advantage and disadvantages too and based on my perspectives I think the disadvantage is more than the advantage.
But I will first start by providing few of the advantage before I talk about the disadvantage.
1, the advance of starting when you don't have a clear discretionary income is that with practical you can figure out your next move, you can figure out the necessary expenses and cutoff the unnecessary expenses.
2, starting early even when things are not yet in place might literally made the person to develop the habit of investment and from there he or she will know how to organize theirsevles for things to work out perfectly for them.
3, starting with small Bitcoin even when you have clear discretionary income can make someone to develop a long term mentality.
4, even for the fact that they don't have clear discretionary income before they get started, it might not stop them from experiencing the market.

The disadvantage of starting without a clear discretionary income is that:
1, for the fact that Bitcoin is volatile, you will understand that it's not appropriate to buy Bitcoin with your survival money because Bitcoin investment is not a get rich quick scheme.
2, you will understand that investing into Bitcoin from your discretionary income helps you avoid unnecessary financial pressures and probably market pressures too.
3, for the fact that you invest with your discretionary income, it will help you avoid debts when an emergency situation occurs.
4, you will also understand that success toward Bitcoin investment is not all about buying but how to build a solid financial structure when it comes to investment.

You don’t have to be confused when you hear that the DCA strategy plays a role in staying active with bitcoin investment regularly, I think @palmyra is largely correct with his statement. All he’s trying to say is that the DCA strategy encourages/ enables the investor to regularly buy bitcoin and stay active by consistently or perhaps persistently buy bitcoin and hold either weekly or monthly basis or most of the times depending on when their discretionary income is ready. with just their discretionary income, they don’t have to wait until its dip before they can buy bitcoin, they don’t have to wait until a huge or additional money comes to them before they can buy bitcoin (Lump sum). In general the DCA is regarded as the most effective strategy of bitcoin investment.

In the aspect of DCA not requiring too much knowledge before getting started he’s obviously right because if you’re investing with the buying the dip strategy as your main strategy you will be needing knowledge of market evaluation, timing and monitoring before you can buy bitcoin. But with the DCA all you need is just a discretionary income to use and buy bitcoin at any market price and hold for long term goal no stress or knowledge of market analysis, evaluation or timing is required here.

You’re absolutely correct, investing in bitcoin without a discretionary income is gambling and it’s also a disaster like a time bomb waiting happen, I don’t really understand how someone would be investing in bitcoin without a discretionary income, and apparently they will be using money meant for their expenses to accumulate bitcoin that would be a very dangerous idea, I’m not a fan of gambling when it comes to investing in bitcoin, and that will never be considered as investing, instead I will have to consider that scenario as gambling, investing with a non-discretionary income is a terrible way and gambling, investing in bitcoin with a timeline of a 4-10 years into the future is more appreciated.

I get what you’re saying mate, and I actually agree with the core idea. Yes investing shouldn’t come from a place where you’re hurting your basic quality of life just to be able to invest in bitcoin. So if someone is cutting their meals, struggling to eat properly, or sacrificing their health just to put money into Bitcoin, then i wouldn’t describe that as discipline anymore, that’s just you putting pressure on yourself, and pressure like that usually doesn’t end well in the long run.
The safer way you should look at it is simple, your essentials have to come first things like food, shelter, transport, and basic comfort shouldn’t be on the chopping block for investing. Whatever is left after you’ve taken care of those things is what you can reasonably put aside to invest.
And there’s no even any real guarantee that suffering now will automatically lead to better financial outcomes later on for you, in fact in many cases, it can even backfire because when your health or focus drops, your ability to earn and stay consistent also drops so at the end of the day, investing only really works when it doesn’t break your life apart in the process.

There are few things we need to learn before we start investing in Bitcoin and foremost is that Bitcoin investment must be for long term. Long term investment is only possible if you have necessary funds available to meet daily and emergency requirements. Without such funds you won't be able to invest for long term rather you will be trading in that case. You need at least 4 years to see results of your investment in Bitcoin. This is why Discretionary income is must in case you want to invest for long term in Bitcoin.   

Investing with a non discretionary income wouldn't work well because such person will sell in a short time frame because he uses money not for investment to invest and when an investor couldn't hodl for 4-10 and above that means he was just trading and gambling all this while you can't use money that you will be needing soon to invest in Bitcoin and call it investment that's trading and a wrong Bitcoin investment system so what should guarantee an investor to investing is his discretionary income, it should be no discretionary income no Bitcoin investment anything out side our discretionary income is gambling.

You don't  need basic knowledge to kickstart your bitcoin investment because that will waste your time from accumulating bitcoin when you should. Learning is a process so you  cannot procrastinate investing in bitcoin until you have basic knowledge instead of wasting time, you can invest and learn Stimutatoulsly. It is not right for you to delay investing in bitcoin cos of basic knowledge, you don't need to know everything to invest in bitcoin rather if you have your discretionary fund available you only need apply commons sense to invest,as you invest you can be learning. This way you wouldn't delay starting your bitcoin investment due to your lack of basic knowledge.