My friend's son keeps hurting my boys. What would you do?

[whatshouldidoaboutthis]

My friend, who I've known for about a year, has two sons aged 5 and 7 and I have two aged 5 and 3.
The two five-year-olds were in the same class at school but started year 1 in different classes.
My friend's five-year-old is a bit of a nightmare when it comes to hurting other children. The last time we went to his house for tea after school, he dugs his fingernails into DS1's arm and kicked DS2 in the stomach. He's always been really cruel to DS2 and I have to keep a very close eye on him to make sure he doesn't get hurt.
Every time we see this child he does something nasty to either DS1 or DS2, or another friend of theirs who's in DS1's class. His mother 'tells him off' in a halfhearted way and threatens punishments (eg the boy won't have any cake after tea if he keeps behaving badly) but never follows through. So the boy knows he can do what he likes and there are never any consequences. She's said she doesn't mind me telling her son off, which I do, but there's only so far I can go. I can't impose punishments on someone else's son and also I feel it's up to her to address her child's behaviour.
The boy, as far as anyone knows, doesn't have any SN but I and lots of others (including his mum) feel he is very immature for his age.
So what do I do about this situation? I don't want to cut my friend off, because she doesn't have a lot of other friends and I think she's a bit despressed and I don't want to make her feel worse. But on the other hand I can't stand by any longer while my children (and others) are constantly hurt by this boy. Also it's difficult to see my friend without the children in tow.
It really is a difficult situation and any advice would be appreciated.

Yes, for his sake I hope it's a phase but I fear he'll just get worse unless someone addresses his problems.

Another valid reason for broaching the subject with his mum!

Well this afternoon was a prime example. The weather was nice so we all, including my another friend and her little boy (who i'll refer to as x) trooped off to the playground across the road from the school. Before we'd even left the school grounds x was crying because the boy had hit him in the face. The mother immediately starts making excuses for him, claiming he must have been playing, telling x he didn't mean it.
X's mum feels the same as me, she's sick of seeing her son hurt. He's been punched in the stomach, pushed into the road, all kinds by this boy. We'll have a talk with our friend by the end of the week. Now I just have to think of how to say it...

Good Luck, Mrs S.

Thanks. It won't be easy.

Well, I did the deed, but not in the manner that I'd planned.
X's mum and I were planning on speaking to the mother when we got to the playground, but on the way there, the boy threw DS1 to the ground for no reason and it was the straw that broke the camel's back.
I told him I was sick of him hurting my son, and when the mother started making excuses for him (he was just playing, he didn't mean it, blah blah) I just snapped and told her I didn't want them to play together anymore, not until her son's behaviour changes, and that I was fed up of having my sons hurt every time they were around her son.
She just said ok, ok and slunk off home with her son, looking upset. And now I feel like a complete witch because she's upset, but she's had a year at least to sort this problem out but has done nothing about it and expected her friends to just put up with her son's behaviour.
I've asked her to go for a coffee on Monday so all three of us can talk about it properly and calmly.

You did the right thing. Maybe she needed a bit of a shock to bring it home to her that it's not all right for her son to keep hurting other children, and your DC wil be relieved that Mummy stuck up for them/
Stay calm when you chat to her and maybe offer constructive advice about getting help with her son's behaviour. but remember your own DC are a higher priority than her problems

I know, but it's hard to tell someone something you know will hurt them. She was really upset.
But I'm relieved that from now on DSs will be able to play with their friends without fear of being attacked. I wish I'd done it a long time ago.

I think you did the right thing, too, mrs s.

perhaps it will have shocked her ds into taking stock of his behaviour too.

it's nice that you will get to have coffee on monday. i hope that goes well too.

Thanks you.
I spoke to her earlier this evening and she said she'd been shocked because she hadn't seen it coming.

Did you have coffee with her today then?

No, her children are ill (as are half the school) and I was busy anyway so will have to wait till later in the week. Thanks for asking.