they need therapy to understand love and sexuality. they need somebody who can communicate on their level, sure they need to know that is it illegal, and they can get into very big trouble if they do it again, but the fact of the matter is that it has happened, possibly several times.
kids at that age may not know it is bad. all they may know is that it feels good, or is something they enjoy.
it may or may not be a good idea to separate them. i don't know the details but it could end up scaring them for life or it could be necessary.
You may want to break up with this boyfriend - his ex is going to make life miserable for your son, and he may end up with criminal charges.
Sometimes parents of daughters refuse to believe that their daughters willingly take part (or even instigate) childhood sex play.
This woman is a danger to your son.
If you have caught them before it would have been a good idea not to leave them alone .Supervison is required in fact I wouldn't allow them to play together at all ......