For the moment, I'm going to do nothing -- except try to manage the problem with ventilation at my friend's house. If problems occur in other places, then I will look into it.

I have let her know what's going on; she is resistant to the idea that there might be mold, but I think a building with persistent (and unresolved) leaks when it rains is an ideal place for mold to grow.

Thanks to everyone for the replies.