Friends child hurting my child

[confuddeledconfuddel]

I am gonna try and keep this brief. Background my friend is a single mum, has shared custody with father. Friend is very vulnerable and I try to be there for her as much as I can and she has had the worst year imaginable. She doesn't really have anyone else to turn to than me.
Her child although seems to like mine (both are 2/3) can be very vicious towards mine. Has now left marks on my child's cheeks and neck on 3 separate occasions. My child is a bit of whinge so a lot of the times when they are together when she cries I do brush it off but this last occasion was extreme behaviour in my opinion (punching, kicking, scratching, pinning to floor and head butting)
I really really want to be there for my friend but feel I also need to keep my child safe. So I am thinking of withdrawing my child from playing with hers for a while to see if things get better with hers. I don't think I can tell her this as it will hurt her so much. She by the way is devastated, was shaking and in tears. I really felt as bad for her as I did for my child.
Does anyone have any suggestions on how I can help my friend. I am doing the right thing in stopping play dates for a while? I feel at a loss and I imagine that she is at an even worse loss.

Sorry- just to add I don't know why some of that underlined and Isla is a made up name I love for the example of talking to dd- as is Polly!

Thank you for your reply @MrsP2015 thinking of it from view point of a stranger it definitely would feel so much different.
I did talk to my child today about not having anymore play dates for a while and my child's response was 'but ** don't hurt me anymore it's all better. I will just tell them no that's naughty' bless them. My child is soo loving and kind and gentle (but sure isn't every mum gonna day that )
I have passed on the hand in hand information, with the way things are with both our schedules it will be easyish not to have play dates for a while. And if we must go see them I might limit it to 15 minutes at a time and build on it and stay right by her side the whole time. I think I have been too relaxed and expected them just to get on like other kids. It was just this latest episode that made me realise it is escalating.
Unfortunately with being a single mum and shared custody my friend only has control of her child's environment for half the week. The other half is not great

That's very sad for the other child however they need to learn what is acceptable & not acceptable around other children.

The last thing that toddler needs is to be punished or taught harsh lessons or anything. They are clearly not coping well with the awful time their mum is going through and is sharing their time with their dad too- inconsistent parenting etc.
Your friend needs support- find out if there is a family centre who run positive parenting classes. Her wee one needs support too- probably love bombed and probably protected and cushioned more from the crap that they are living through.
Don’t meet at softplay- meet places where you are always close on hand but don’t be all shouty and using the word no- say “we play nicely and are kind” rather than “don’t hit or don’t do that” and model calmness. Your wee one sounds like the perfect friend- so take her lead. Yes protect your wee one, but by being closer to this other wee one and not pushing them away. They’re only babies really. And is the mum is struggling then her wee one will be picking up on all that tension too. Very sad.

You sound like a brilliant friend and parent, you understand what's happening and why and I think you've already got this. Your friend needs support and you can give it in more ways than one. Lead the way with the children and discipline, and your friend will learn from you and maybe find the strength she needs from having you help her with this. My close mum friends and I all play the adult role with our children, when together, and it doesn't matter who's children are whose, if one hurts another, an adult will step in and the other adults are fine with this. Obviously you need to put your daughter first and if supervised and nipped in the bud I'm sure you can keep her safe x good luck, I'm sure you will do the right thing x