I don't understand the story. Are you saying that he set up his wallet at home, left the phrase on his table and another family member found it, knew what it was and used it to steal his money?
Is 1k a lot of money to your friend, because the story suggests it isn't. I find $1k to e a decent amount of money, enough to buy a computer or 2 sets of tires for your car (summer and winter), or a pistol, if you're into guns... I wouldn't invest 1k in something that I don't understand.

Your friend's problem is not lack of security but negligence and carelessness. You don't need proper security to put your seed phrase in a "safe" place, like inside a book, or an encrypted file on your computer.
In my early days when my bitcoin wasn't worth a lot of money I used to keep my private keys in renamed txt files hidden somewhere on my computer. I'd take a normal text file, change the extension into something like "infinity.dll" and hide it somewhere. That's not proper security, but still many times better than some people, who store these on gmail accounts, cloud servers, or password managers.

upon creating a wallet the app will remind the user how important the seed phrase is afaik. i think your friend knows how important it was so he wrote it. it however just flew out of his mind how to keep it out of someone else sight. when it comes to money, it shouldn't be left alone on top of his desk.

it took him 2 months to come looking for it.  a person who knows what a seed phrase looks like may try to import a wallet for himself to withdraw so he best be cautious about it.

if he is still logged on to his trust wallet, can't he see it from there to write it again?

Security is very important, but I will tell you that this story looks frictional because if he left his seed phase on the table and lost the seed phase and later found it again just to login to find out he lost his bitcoin, defiantly the bitcoin was stolen by one of his family members or his friends. This happens as a result of his carelessness. If not, how can you buy bitcoin worth two thousand dollars and keep your seed phase careless? So I will like to recommend and add to your post to say that instead of encouraging a beginner in bitcoin to know more about the security of his wallet and his private key, he will also not be too lazy to learn and perform educational research, and most importantly, he shouldn’t be as careless as your friend who misplaced his private key intentionally.

Op I think you need to take your time to go through your story then consider how interesting it is because I find your stories to be very far from the truth,  I mean how will someone wake up and decide to buy Bitcoin that is worth $2k without knowing what seed phrase is all about? The most funny thing is that he don't know anything about seed phrase but he wrote it down and kept it on the table why did he keep it on the table and how does he know that the seed phrase needs to be written down since he has no idea what it is about. I think op needs to come up with something reasonable because I believe technology is everywhere now even a layman will understand the importance of seed phrase before getting a wallet.

Firstly, when it comes to online platforms that require most of our personal information or for business or invest purposes, it is good to observe even the basic security protocol, which could be just using passwords, authentication and other notifications.

For a BTC or cryptocurrency investor who has some funds in a wallet or intends to acquire some, using good wallets mostly offline wallets with key phrases, should be safely  kept secure from prying eyes and criminals.

Without a secure wallet or secure asset location, there won't be a need to want to keep up with an investment plan or even portfolio, because why bother when hackers or anyone close could easily have access and make away with the funds?

FYI Private keys and seed phrase are not the same thing. 

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$2k worth of bitcoins lost due to negligence is disheartening. It’s even more sad that your spent his savings on bitcoins only to lose it all two months later. Two things I noticed from this tale;

First your friend isn’t very knowledgeable on bitcoin yet he spent his savings investing in bitcoin. Which makes me believe he didn’t follow the “invest only what you can afford to lose” rule.

Secondly, $2k is not a small amount of money and must mean a lot to your friend. Unfortunately he made the dire  mistake of not storing his seed phrase properly. I think he only wrote it down because the wallet required him to write it down and type it again when creating the wallet.

What are we talking about here?  This person does not need a backup (seed) phrase to access his Trust wallet. Unless he somehow lost his passcode or his smartphone completely, and  that would be really stupid if he had deposited a significant amount of money into such wallet.  

Also, the backup phrase is not the same thing as a private key.  Those are two distinct terms that should not be used interchangeably.

OP, you must understand that we are not talking about security when you leave your seed for the world to see.
It's like if you had a car and invested in GPS tracking and the best alarm system out there, but you'd leave it outside unlocked with keys in the ignition.
You could have the best hardware wallet and a security safe to keep it in and then leave a piece of paper with recovery seed outside the safe.
No security measures will work when you're stupid.

Yes, it's $2k worth of investment, how can be someone as careful as the OP's friend? For us it's a huge amount of money already, and others it could be just a dream that we can invest that much and then protect it at all cost.

Although there's no one to blame but your friend because of his carelessness or probably he is ignorant. That's why we said that bitcoin investment is a BYOB (Be your Own Bank). So at least you should learn and know how to protect it. And talking about security, just search everything about it in this community and maybe you can give a piece of advice to your friend to not make the same mistakes if ever he will invest on bitcoin again.

Your friend is careless and he is careless as ihis habit. I don't need to know a seed phrase is important to keep it safe and in a secret place. I do this with my bank accounts and I am sure will do the same with other accounts that store my money.

If your friend does not know that a seed phrase is important, why he wrote it down, it is unnecessary step if seed phrase is useless.

Security checklist
How to back up a seed phrase?
Wallet basics