Kids experiment with this at a much younger age when it is innocent. Usually around 4 or 5. At 7 I think he has begun to know that this is not just playing around and innocent. not that he knows sexual things but I am sure he knows it is not appropriate. I would tell your friend that she is to sit her son down and let him know, very sternly, that this is NOT appropriate. I would also tell her daughter that her brother is NEVER to look at, touch, or take pics of her private parts. I would tell her son that until he can behave like a respectable little boy he will not be allowed to play with these other little boys again. All kids are different but I know if I told my 6yr old son in a very stern voice that this is not appropriate behavior and he is never, ever to do that again, he would not do it. If it does continue, she has a bigger problem on her hands and may want to seek counseling for both of her children as the divorce is very hard on them. Also, I wonder if he has seen or heard something on this issue. She should monitor his tv and friends more.