Well, if your friend has been well-informed of the risks, the losses, and the potential headaches that may ensue with investing in bitcoin, I don't think you shouldn't absolve yourself of the pressure, after all you're not Jesus who could control the economy of bitcoin just to look impressive to your friend.
And he strikes me as someone who's smart enough to know what he's doing, he's probably thought about investing in it too, but is just waiting for a push from his friend circle. So I assume he pretty much knows what he's getting into, and if he's a good friend, he's not gonna blame you for any losses he may incur in the future.
To be honest when looking at the context that indeed op's friends are in 2021 for that matter then I think 2 years is definitely a good learning and of course I think he has learned from all aspects including from the worst possible.
Now, when he feels sure about it, there's nothing he can do because he himself chose his own way.
Warn bitcoin that he has been around since 2021, I don't think he needs to be reminded because indeed a period of approximately 2 years he should be quite aware that choosing an altcoin is the wrong decision but in this case he is still not in bitcoin doesn't mean he is unaware but rather unwilling because it's been 2 years in the altcoin of his choice.