I also think it's wise you kept your son comfortable when you first talked to him.
I don't know how close she was to them.
I would try very hard to get him to keep his sexual activities to himself.
Consider whether or not you want this to happen to another woman when you could have done something to prevent it by reporting what happened to you.
Does anyone have any insight about the predicament? Have them go with you to the hospital, just to make sure everythings okay with you.
Earlier today I was taking a box of Christmas decorations out to the garage and when I entered, I discovered my 11 year old son and his friend with their pants down, performing mutual masturbation on each other.