J.,
What happens when the boy is doing these things? What's mom's response?
It may very well be attention-getting behavior. However, your friend needs to put her foot down about it. No chatting, no explaining... he's old enough not to be doing this. I have seen this dynamic at a playgroup, where no one wanted to confront the mother and she was always 'explaining' "oh, sweetie, we don't hit/hurt" etc. She still hasn't dealt with it. (I stopped attending the playgroup when her child began this behavior at about 3 or so. Three years later and he's STILL doing this. It's terrible.)
And just so we're clear, YOU don't have to put up with this. Even if mom is not attending to what's going on, you can be very clear with "STOP RIGHT NOW. YOU ARE HURTING HIM." Firm, clear voice. Do everything you can to get between this kid and your child, pull your child out of harms way, pick up and go if necessary. I know it's not 'fair' to your kids if they have to leave, but this will also send a message to mom. What you are describing is very scary behavior and believe me, it won't stop unless it's firmly addressed.
I'd make sure future playdates are not at your house, so you don't have to insist they leave. "When I hurt my friend's brother, she goes away too" is a great lesson at this age. You can't solve the problem of this behavior, even if it is attention-getting, only his parents can effectively address it. You can, however, consistently make boundaries and teach in this way-- that YOU won't stand for it. It's really all you can do.
One other thing, J.-- I know your group practice is to discipline in the moment, no matter who's kid it is. In this situation, I think she needs to see you in that moment. I'd take him back to mom and tell her "He needs to be with you for now. He's hurting other kids/He just did XYZ and I need to be caring for the other kids." Just leave him there with mom. I'm sorry this is so hard. I know she is at a loss-- the other mom was too-- but putting it back on her is actually a good thing. (The other mom I refer to from my playgroup is *still* at a loss and needs to develop some backbone with her son. Maybe being in a spot, socially, will make her angry enough with him that she will start correcting him and disciplining him immediately instead of expecting others to do it.)