Anyway he owns and does long term rental of maybe 20-30 apartments mostly in Munich (not airbnb or similar). No problem for him buying additional apartments and houses in very expensive places across Europe on spur (or recently Egypt). Also some old sports cars etc. Yet often a penny pincher when it comes to investing into relationship.
The weird part is, when asked what he was doing before becoming a rentier, 'he had some manager job', in Germany. Now I haven't lived there specifically but I know net salaries don't go that high if you work for somebody else, and taxes and social deductions are brutal. This ain't US where 10x performance can give you 10x salary if you do it right and are lucky. And he ain't a C-suite material.
He keeps claiming its all just his own effort but I just can't calculate a realistic path where even a highly capable person can get in their early 40s such vast net worth from 0 (I could grok some lucky exponential path via airbnbs but thats not what he does).
This is a very conservative investment path of ending up with maybe 2-3 apartments if you really push it hard and do all the right choices at the right time. So he is either laundering money for some mafia/gang (but certainly doesn't seem like so) or got a small help from family initially and refuses to admit that.
Nobody thought about that until I came along and just wondered about those numbers a bit.