You could just point them over to the marketplace section of this forum, to demonstrate that Bitcoin can be used as a currency, and isn't just a "investment currency". Besides, these days you can pay for almost anything in Bitcoin with payment gateways like BitPay, and the like. If you want some examples of mainstream companies that are accepting Bitcoin, take Namecheap, Microsoft, and once upon a time Steam did.

There's also jobs which pay in Bitcoin. You can find some of them listed on Angel list, and probably elsewhere. If you've taken a look at the marketplace section, you've likely found a lot of things which Bitcoin can be used to purchase.

The only really thing that Bitcoin hasn't touched upon, at least in the mainstream is groceries, and property. There are ways to use your Bitcoin to purchase these, but no real direct, and within the mainstream scope.

At the moment you might consider Bitcoin to be more an investment opportunity, than its usefulness as a currency. However, I'm not sure why that would be a bad thing, and doesn't change the fact that many people use it as a currency just fine, but specifically to you or your friend it might serve better as an investment. The best thing you could do, is ask your friend what he would like to purchase with Bitcoin for it to be deemed a currency to him. I would wager whatever his answer is there's a way of going about it, maybe not directly in the mainstream companies, but there will be an alternative.

I share your opinion, I even showed him some of the points observed here.

However in the country where we live, the idea that Bitcoin or cryptocurrencies only serve to make an investment and obtain a financial profit is a reality, almost always you buy cryptocurrencies to sell yourself next and have more government money (FIAT).

I pointed to the growth of decentralized trading platforms as one of the great advances that we are making at that time, where we are no longer being held hostage by centralized systems that aim, exclusively, to attract partners and owners of these developments.

In countries where the government currency is not so accessible by the majority of the population, the use of cryptocurrencies has become a reality, however as noted by our friend Welsh, there are gaps that still need to be filled, for example, the basics can reach the table of the most disabled population.

Between decentralized exchanges and the use of Bitcoin, I believe that the biggest sin is still having to resort to government currencies to buy food.

How do you see that this could be resolved in more future times with the use of decentralization?

His fiat money in the bank can be taken by the central authority since it ain't decentralized. If he has millions and authorities find some faults on him, he'd be scrambling to find here to get money because he can't withdraw his funds anymore.  Decentralization is freedom.

But I wouldn't be arguing with friends, it's best to just argue with someone who isn't your friend. Even if you win or lose the argument, you'll about to lose one if you do that.

And I actually wouldn't refute it tbh. Most of the late adopters of Bitcoin are in it for the financial profit/investment they could get, they aren't really using crypto supposedly for what it is for. They don't even realize what Bitcoin being decentralized means (probably) and just hodl it and hope for the best. Though can't exactly blame them, the progress that was mostly in the know is about Bitcoin being an asset after all, so maybe just give it time is the best. If you explain everything that was what Bitcoin is, and yet they still refuse, it just simply means that they don't understand what it really means. They don't understand what decentralization is, what a trustless system is, and what it means to get out of the control of a central authority (which is ingrained in the minds of most people imo).

I really can't comment on this because I haven't dug deep onto other currencies besides the USD, but I could see how you think that.

While there's a lot of fuckery with fiat currencies in general though, I'm sure you'd agree that some countries handle their currencies a lot better than others that are currently already underwater. They're probably all going to end up the same way though, it's probably just that other countries are better at delaying the inevitable.

When you look at cryptocurrencies then obviously your friend is correct because first of all bitcoin is lost among 10000 cryptocurrencies and secondly out of all the altcoins only a handful of not so popular coins may have the potential of being actually used but all of them remain unused and serve only one purpose which is being traded.
But if you look deeper you will realize that you should ignore altcoins and realize they are like a worse version of penny stocks. Then you only have bitcoin to look at and you realize that it is widely used as a currency too among bitcoiners. There are lots of businesses both online and offline that are accepting it as payment. There are even countries that have accepted bitcoin as a currency not just a commodity. A quick google search of "where can I spend bitcoin" will give you a lot of results too.

Exactly, you let them come to you and not the other way around because that is how most scammers work their way into duping people and it is the instinct of a lot of people to avoid people that are annoying or nagging them even if it means helping them. But you can always try approaching them first but do not force them, let them know the basics and just let those words marinate in their thoughts and have them decide if they want to do it.

Not to mention that it will be convenient for you that they are doing their own research about bitcoin and other bitcoin related stuff because you might be wasting your time explaining to them and then they are just like I don't want to do it but in the case of DYOR they are responsible for the discovery so they will not blame anyone when they do something stupid or they don't want to get in on it.

I am going to differ a bit from most replies and I am going to agree with your friend a little bit. As of today, Bitcoin is succeeding as the best store of value in the world. As such, it is an investment, and in some cases, like gold, it can be used to make payments, but this is not its main function. This differs from Satoshi's original idea, but I don't care, the fact is that Bitcoin is succeeding and that's the important thing.

There are several things Bitcoin and Blockchain Technology can used for. Outside investment and paying of bills. The gaming platform are enjoying the decentralization brought by this awesome tech. The academic sector, Auto-mobile, Health sector etc are not left out.