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+.

I don't believe it. If he could guarantee it, he would instead borrow from a bank at 3-5% as much as he can (potentially using his house as a collateral) and invest that. Besides, at that rate of return, other investors would flock in, including fund managers, and saturate the market.