This is a scary situation. I would even think of talking to a lawyer before I did anything. This is a terrible world where the victims get blamed. My intuition is telling me that something is seriously wrong here.

I might laughingly tell the girl's mother, "Your daughter keeps taking her clothes off when she's at my house. What's up with that?" And see what she says.

I'm glad you are talking to an expert. Please let us know what happens?