Re: Defiant little girl...I have to tell my friend that her DD can't play with DS any
That isn't really even normal behaviour for a 1-2 year old, IMO. I mean, maybe the diaper cream - kids do love to open things and play with whatever is inside - but even that, at her age, she would know that it's not okay and she shouldn't do it.
I wouldn't want to have her over at my house anymore, and I wouldn't be allowing my kids to go to their house either - mainly because if they handle her that way, they're going to be the same with your kids... and I wouldn't want that behaviour modelled or positively reinforced with my own kids.
I don't think there's any way to mention the possible issues with her behaviour. It would be great if her parents saw the differences in her and would be totally different if they mentioned it to you or asked you for advice - then you have an opening to talk about it and offer help. But if you bring it up it's going to come across wrong to them.
Just deal with not having the kids together for the time being and leave the rest to her parents.