I would tell the parent what happened (in a way that is very carefully concerned and not at all accusatory of her parenting skills or anything like that). And maybe ask if she has noticed anything odd with her kid. It might be that she is slightly worried but is scared to talk to anyone about it for fear that she will be blamed. It might be that she thinks such behavior is normal.

It is hard to know what is going on... But I'd say that yeah, the kid doesn't sound too happy and it would be nice if she could get some gentle help in learning what is and is not appropriate. I can't see that it would be particularly helpful to start running through a list of all possible abusers, because you don't know that she has been abused. Though her behavior is, of course, suggestive.