Eeeek! Probably was kinda (dare I say it) common cheeky teenage boy behaviour...maybe...but I think it is VERY wrong. I would just mention it to the mum, not accussingly (sorry sp?) or anything just say that something like that might have happened and it made you (if it did that is) uncomfortable as he was a guest in your home.
If I was the other mum I'd want to know if my son did that.

Poor Isla's mermaid! Get scrubbing FJ!