I'm sorry to hear this - how horrible for that boy NOT to have rules and discipline.
I was in a similar situation - here's how I handled it:
1. When the playdate was at my house - I made ALL the children follow my rules - PERIOD.
2. If hitting or fighting started over a toy - the toy is taken away and put in time out. The TOY, yep, but it seemed to work.
3. If we went to another's home: I would tell him child loud enough for the other parent to hear "just because other children aren't behaving, doesn't mean you can act like that."
The second ANYTHING got violent - pushing, hitting with a toy - I would stop it - I didn't care if I offended someone else. But that's me. I would also discipline someone's child too. One parent complained once and I told her that if she wasn't going to do anything about her child's behavior - I would. She didn't like it - because her little Johnny was perfect and just being a boy. Well, I didn't see the other boys acting that way. She was embarrassed and didn't come back.
I hope this helps you!!
I do believe that EVERYONE needs rules and guidelines.