I made a new friend when DS started school and was friendly with her son.
But her son was always hitting DS, stabbed his face with a fork while they were eating lunch then bullied DS to make him stay quiet about it, hit him in the arm with a plastic plate, locked him in rooms including one time when he wanted to go to the toilet, lied to him about DH shouting DS to go home, demanded money from me to buy sweets.
DS started to avoid being alone with him and he started to make up lies about how he had some sweets in his bedroom and DS could have one if he went upstairs, then he would hurt him when they got there.
He would also make a mess, tip all the toys out without actually playing with them, break things or damage them a bit etc, whereas DS has always been very careful with his things. I could put up with mess, wasn't happy about things being broken or damaged though.
It didn't take long for us to realise this boy was a bit boisterous and violent when they played together and I started to watch them very carefully. But at first we didn't realise quite how bad it was.
His mum would sit there quite happily saying "that's not nice...say sorry" but without really looking or doing anything to really stop it.
While we thought it was just boisterous play I started to tell him off myself. "Don't do that again or you will have to leave", "Don't hit him, he doesn't like it and he doesn't hit you", "We will go home if you hurt him again", "we don't play like that here, it's not nice to hurt people/break things on purpose" that sort of thing.
His mum didn't like it, especially when I followed through with whatever I had said.
It's quite hard to say "I think your Mum needs to take you home now, come and put your shoes/coat on" and turf your friend and her child out of your house, but seriously I would rather do that than watch DS get hit in the face with one of his own toys and feel upset and scared in his own house.
She was very surprised the first time I gave her their coats when her son hit mine for the second time in half an hour.
I also told DS to come and tell me if something happened and to tell the other child's mum if he could. Or tell the teacher at school and me when I picked him up. (Because school are weird like that. They tell parents about the smallest bump but didn't mention the time someone put a hula hoop around DS's neck, dragged him to the floor and left him cut and bruised.) And I told him it was fine to say "I don't want to play with you if you are mean to me/if you hit me" etc to someone.
When we realised it was more than boisterousness, after a visit to their house that had him hit DS twice, lock him in the kitchen and then in a bedroom to stop him from using the toilet and then DH overheard him tell DS that DH was shouting him to leave before hitting him and saying "Go home now" to him, we decided enough was enough and we don't see them now.