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 so disagree with the op . I have a friend he is a retired banker. He is wealthy and he is 77.
He has well over 20 million usd. We mine in his office just for fun.
So if a 77 year old worth over 20 million usd is willing to mine and we are still trying a good property to turn into a mine the answer is wealthy people do have an interest.