Hey OP, if you don't want to buy Bitcoin any longer then it's your choice ,that won't make others to stop buying it, there are lots of people who will continue buying it and everyday lots of people keeps getting in.
Moreover FYI, what you have said here is wrong, of course I have heard about the 51% attack or whatever they call it, but that's not enough reason for me to quit. People keep on imagining things that are never even going to happen. A lot of people come up with these as a way to discourage others from taking part in buying Bitcoin.
let him with all his thoughts. people will not change because of other people's principles, so when he doesn't want to buy or save bitcoin and think it's rubbish, he won't use it. Well, even if it's like that, I'm sure he's still trying to collect other crypto assets. I don't believe he will take his hands off this.
even more so if he knew that there was a halving up front. He will leave it if he does not know the potential of halving.