Well you are absolutely correct in that he may be a victim himself. I wouldn't be so complacent or shall I say confident and comfortable that nothing could be happening at his friend's house. Unfortunately, most of the time when people find out someone was a predator, they would have NEVER thought that about them. It happens and it's very scary! I think you are on the right path by looking into getting him to a child psychologist as they are more capable at pinpointing the source and redirecting his behavior. Best of luck to you.
Thanks for your response. I did praise my daughter for telling me and I did tell her how important it was to always tell me and Mom if anything like this ever happens so I guess I did something right...we are looking into child psychologists now for my son. My fear about not finding out where he learned this is that there could be someone actively showing or abusing him... He is out of state with his Dad a good part of the year so there is no way for us to know who it could be or if it is his Dad or something. He only goes over to a couple friends houses here and I know the parents and children very well so I am pretty sure that it is not someone here...
First and foremost, you need to praise your daughter for telling you what happened and for telling your step son that she didn't want him to do it again. Reiterate to her how immensely important it is that she ALWAYS tells you if anyone touches her private area and that no one is ever allowed to touch her.
I wouldn't concentrate so hard on where he saw or learned this behavior. Focus on preventing this from happening again or escalating to more extreme acts. I would talk to a few people, family physician, school counselor, daycares, etc and get references for a good child psychologist. You need to nip this in the bud way before he even approaches puberty. I would not let it pass. A childhood friend of mine was molested from the age of 5 till her teens, being forced to perform oral sex on her older brother and cousins. You definitely do not want your daughter growing up being violated.
Try not to lose your temper with your stepson, I think a calmer approach will be the key. But then again, maybe it is good that he saw how serious this is. The behavior definitely isn't normal and it definitely isn't acceptable; this is something you will probably have to monitor for years to come just so that you feel confident that your daughter will be okay.
God Bless you and your family.