1) My daughter has had some "alpha" friends- funny because she tends to be that way too, but the rare few end up being more controlling and have their rules. In that case- it is what it is- I take it as an opportunity to teach my child to think for herself, push back and not always be the follower. It is a good life lesson and there are going to be instances like this a lot. I also encouraged lots of playdates with other kids so she wasn't always in this situation, but never stopped the playdate.
2) if that happened, honestly? I am not so worried. They get introduced to so much when they enter school. My K daughter takes the bus everyday, and came home and asked Mommy- what is the "f" word, "s" word, and "b" word. I was like WHAT?! She said kids on the bus were talking about it.
Unless someone was really mean/hurtful to my child, or I felt that the house she was visiting wasn't safe with good parents, I take it all as growing up.