Been there done this too many times! It is like a switch isn't it? I have a little boy who I've just gotten through this phase now but he could have won an Oscar, I swear! I called him my Jeckyll & Hyde boy because he turned into a monster I didn't even recognize when his Mom was here, then switched it off in a second when she left, OMG. But he's 3 now and I finally got through to him by talking to him and making him understand. When he was 2 it was insane and his mother didn't know what to do so I would take him and sit him for a timeout but that just made him worse. Just keep trying and enforcing your rules and let the parents know that it is still your house and your rules and that you don't appreciate the behaviour. Try to get them to leave as quickly as possible.