Well, that sure is frustrating. I sounds more abusive than sexual to me though. The making the 'fart' noises kind of indicates she is trying to be gross rather than be sexual with her. That's if it happened which I guess you are determining.
WHO sent you the message?
Your daughter says it didn't happen. who is saying it did? And is anyone else confirming it in any way?
I think if it were me, I'd have a talk with my daughter that she is to tell if someone gets naked and sits on her. Period and that it is a rule that this is not allowed. That she doesn't touch naked friends and they don't touch her. Period. Rule. And then the other family should say the same to the 12 year old. The "kid in a 12 year old body" part too, remember, 12 years old IS a kid still. She's still a child too. If both know this isn't allowed, whether it happened previously or not, it may never happen again.
If I thought from anything you wrote that this was a sexual situation and your daughter was being molested, I'd speak up. It's inappropriate but I think it could be managed. Child on child molestation is different. That's often a sad situation of the child doing the act to another that had previously been done to them. It's a vicious cycle. If your child were actually molested, I would say working with a child psychologist would be the best choice. But I don't get that sense here.