I'm trying to figure out what percentage of a balanced investment portfolio should go towards rental real estate, but I'm having a hard time finding reliable sources of advice on this question.

I have a friend who invests in rental real estate, and he says he can give me a guaranteed 10% ROI if I invest $10,000+ with him, or 15% if I invest $100,000+. From looking around online this does indeed appear reasonable - rental real estate often gives much higher returns than this, so it sounds reasonable that he can guarantee a lower rate and then either pocket the remainder (his reward) or pay up the difference out of pocket (his risk). So it sounds like a pretty decent investment as far as I can tell.

But I don't want to put all my financial eggs in one investment basket - I'm not an expert, but I've always heard that diversification and a "balanced portfolio" are the names of the game. My question is approximately what percentage of my assets should I put into rental property investments like this vs. e.g. a Vanguard targeted retirement fund. As I said, I'm having trouble finding reliable sources of advice on this question.

Anybody here know anything on this subject? Anybody know somewhere I could go to find accurate, reliable, and unbiased advice?