this is what parks are for! if you have different house rules to your friend and her child, and things are stressing you out, arrange park meetups, not home meetups. Easy at this time of year, too.
I think familiarity breeds contempt. So yes, I agree it's something that just happens even to the two most well behaved children . Three full days in a week is a lot of time to spend with other friends, and IME when this happens, bad habits and playing up creep in. So limit the times you all meet.
Step in now and change the way you meet, so that you don't build up resentment between you and your friend.
It might not be that the other children are especially bad, just that they all get overexcited and have a slightly bad effect on one another.