It would be a really bad business if their rent could only cover interest, taxes, insurance, and maintenance i.e. expected change in equity is 0. If it covers principal as well, then it's merely a risky business as expected change in equity is positive (since the mortgage amortizes), but downside (foreclosure) is huge and likely.

In either case, it might still be a good _investment_ if the value of their equity appreciates - but as the other commenter notes, that's a very risky and inefficient way to get exposure to real estate prices!