Wow people. I have several friends whose kids I find difficult, but there are so many things you can do before ending the friendship, if it's a real friendship.
1. See the mom without the kid.
2. Correct the kid yourself--not by calling them terrible (!!!) calling them names, but "Johnny, no hitting," take away toys if they are hitting someone with them, say you are going to have to leave their house or they will have to leave your house if he can't treat the other kids kindly, etc.
Only if you can't do #1 and the other parent reacts badly to #2 would I actually end a friendship.