My Neighbors 3 year old daughter is displaying ual behaviors?

I went through a very similar situation with my 2 year old daughter. If you do decide to let the little girl come over still then i would always keep an eye on her while she's playing with your son. Make sure if you see it start to happen stop her and tell her that is not how young ladie's should act and so on, you know. I wouldn't leave them alone cause most definately she will keep doing it and teach your son. He is at the age that he sucks in everything like a sponge and might try it on another kid (ahhhhhh!). I let my daughter play with my next door neighbor's 2 1/2 year old son alone and he taught her how to undress (which she never did before) and where to touch herself, among many other annoying, bad things. We would leave them alone for 5 minutes and they'd both be undressed running around touching themselves. One time when they did this he even peed on her! I was horrified. My neighbor said she didn't realized when she put on some kiddie movie on cinemax in his room when she'd go to bed that after the movie, soft-core would come on and that's were he got the idea. Weather or not that's true we don't talk anymore and the kids don't play together anymore. We live in military housing and our units are attached and we still don't talk. It makes it akward but her kid has done nothing but teach my daughter bad things and i don't care. Because of her parenting I have to duct-tape my daughter's diaper on or she'll take it off and run around touching herself. I truely believe she wouldn't be doing that now if I had nipped it in the in the beginning and not let her play with him unsupervised. That is my experience with this type of situation. I hope I helped! :)