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.
because basically financial issues are very sensitive to talk about, so we don't need to bother digging deeper into other people's financial problems,
If they don't own Bitcoin, it's possible they are already too rich and don't want the headache of seeing Bitcoin price movements fall very quickly and take months to reach a new ATH.