Friend's aggressive child

I have a 5 y.o. daughter who plays after school with a friend's son, who is turning 4.  He displays inappropriate aggressive behaviour towards my daughter when they get together:  jumping on her back, kicking, hitting with objects, choking, just an all around nut.  His mother always has an excuse for this behaviour: "he's sleepy" or "they say little boys hit little girls they like", which is bull. I've raised my child not to hit but have recently given her instructions to whale on him if he hits her.  Nobody want to be a punching bag and his parents do not seem capable of addressing this issue.  His mother say he doesnt act this way in school, which only makes me feel worse, like i'm offering my child as a sacrifice for his aggression.  Its to the point that if the parents and i did not have history,  this little boy would be on the "Do Not Call" list.  I want to address this in a way that clearly states where I stand on this, without alienating the parents.  They always try to explain away his behaviour and I believe that kids have to respect others personal space regardless of how sleepy they are!  Suggestions?

[specialmom]

Ask his mother prior to the playdate if it is a good day----  is he sick, sleepy or okay.  I wouldn't tell your daughter it is okay to hit back-----  I would instead tell her to use her words first "Stop hitting me.  It is not okay."  Then have her tell the boys mother "X is hitting me, please make him stop."  and then if it continues, you step in and say, "okay, it looks like X is having a bad day, we better go."  

Too often, parents will leave a 5 year old and 4 year old to play while they chat.  I guess that in this scenario, you can't do that.  This boy may need some extra help with play skills.  Get a game going in which you play with them too and teach him how to interact.  I know it is his parents job but since you seem to want to continue the playdates---  then get involved in helping him.  Ask him questions like----  did you mean to hurt Y?  He'll say no most likely, and then you say, well you did.  

If your anger shows to your friend as it does here, the playdates will come to an end anyway due to the tension.  Good luck.