Eh, I'm pessimistic because my best friend trooned out. None of us could do anything, he alienated everyone in his life and went totally insane. He did calm down and reconciled with everyone, so it worked out in the end, but yeah, I'm really pessimistic about anyone trooning out, it never works out.
I'd just try to get them to come to terms with, just because they want to feel like a woman, doesn't mean they should mutilate their body for the sake of it. Tell them, hell, show them that most troons look really fucking ugly, and the "good looking ones" trannies use to lure people in with are a very slim minority who probably started HRT when they were very young. Try to get them to see, without belittling their feelings, that trying to become a woman is just as promising as trying to turn into a dog. Try to get him to find other avenues of fulfilling his desires, like, catfishing and roleplaying.
At the end of the day, reality sucks, we're never our ideal persons, and this is why we have escapist media like video games and anime. Tell him not to conflate wanting to have a different body with wanting to ruin it for the sake of social brownie points or a vain attempt at feeling better.
Most importantly you can't try to tell them they're wrong or that you disapprove of it, merely that you're worried they might make a mistake they'll regret. Even if it's just for attention, you gotta let them know you're on their side and you sympathize with them, but that what they're considering is not going to make them happy. The 41% is proof of that, and that number is not from social stigma.
Of course, if they don't listen, be prepared to cut them out of your life. You don't need that kind of person in your life, basically trying to force you to roleplay their fetish with them. Don't threaten to leave them if they do consider it, just be prepared to do it if they do.