Had a similar experience years ago at a parent/child group. Mother who,thought it was her time to socialise and sit and chat, ignoring her child's behaviour.
Her DS used to snatch, hit, kick and punch when he wanted something or didn't get what he wanted. She just let him get on with it. Whilst I felt sorry for her DS, there was no way he was going to be allowed to hit my DS (who was younger), push him over and snatch the toy he had or smack him on the head with toy cars.
I used to keep a very close eye on my DS and the other child and as soon as I saw him making a bee-line for my DS, I would be ready to step in. A loud, firm voice normally did the trick 'DO NOT HIT, THAT IS NOT NICE' or 'DO NOT SNATCH THAT IS NOT NICE'.
After a couple of weeks, the DS's Mum asked me why I kept telling her DS off for not being nice, as it upset her. I just responded and said 'it upsets me when you consistently allow your DS to hit, punch, push and smack and sit drinking tea ignoring it. I will carry on telling your DS not to behave like this, until you decide to be a parent'.
Suffice to say, she never came back. Other parents were over the moon.
It's such a shame for the children when parents ignore this type of behaviour. If nothing changes, these poor children grow up into bullies and generally very unpleasant - when it doesn't need to be like this.