If he is already rich enough, he may not need bitcoin, although I find your argument too simplistic and I would like to know your friend's level of 'rich'. Do you know approximately how much wealth he has?
Anyway, it is a bad idea to try to change people's minds on financial matters.
I do not agree totally with this assertion that he is already rich that's why he doesn't want.to invest in Bitcoin.
If I may ask, is Elon musk not rich, or did he make his money from Bitcoin? The obvious answer is no! There are many rich people who are still holding Bitcoin.
So, I don't want to believe that he isn't investing because he is already rich. I think this person has personal reasons why, probably he doesn't have faith in it and don't believe in it.