At a football tournament at the weekend. There are two boys who play for my ds's team that can be disruptive. One of the parents drops him off and leaves, the other is very loud and can be heard telling him quite loudly that she "will batter him" or "knoc his head off" if he doesn't stop being naughty. He completely ignores her and carries on doing whatever he's doing!
We were waiting to play our final game and these two were fighting and throwing things at each other and the car park was right next to our pitch. A couple of the parents including me tried to reason with them to stop, they ignored and carried on. I could see mum chatting on the other side of the pitch so I asked them again to stop, they were very rude so I shouted at them to stop now before one of them got hurt. They laughed so I shouted over to mum to come and sort the situation out.
She came storming over shouting at me saying "who the fuck do you think you are calling me over", and continued to be verbally abusive to me very loudly. It was clear we weren't going to have a productive conversation so I simply said "OK, next time I will leave him for you to deal with" and walked away. She told me that I was a stuck up cow and that I should "fuck off" All of this in front of 7 year olds!
Since Saturday I've been thinking that maybe it wasn't my business so I should have let these two boys just continue with what they were doing and not intervened since the boys mother clearly wasn't bothered.
If you decide to leave them to it, and one of them really hurt the other/fell/hurt someone else you would now be kicking yourself wishing you had intervened. I agree you were not going to win in this situation, the boys weren't upset as they were laughing, she sounds like she needs some firm parenting advice (or a kickup the arse)