Back story -very good friend who has 7 yr old twins. I've known her for about 10 years and know how desperate she was to have them. She used to be a child-minder is someone that kids tend to adore. My DDs love her.
DTs are a real handful and always have been. Cheeky, bad-mannered and so willful. They have been through a lot as friend split from husband a year ago (not v amicably) and it's been hard on them all.
Anyway, friend has a temper. At the moment, she is having issues with her neighbours giving her stick about the children being too noisy, her divorce is imminent and she's v stressed.
However, yesterday we were walking home together after school and DTs were playing up as usual. They started bashing snow covered bushes on very narrow path, spraying us all with snow. Friend said "stop it" at least 4 times. One twin then ran on ahead but other twin then completely covered the dog in snow. I should have realised friend was at boiling point and bit my tongue, but I blurted out, "oh, that wasn't nice, poor dog..."
Friend then hit twin hard in the back with her school bag, enough to make her jerk forward but little madam turned round and said defiantly, "didn't hurt!" Cue friend completely losing it, slapping her DD on the face 3 times (my DD said twin managed to dodge and didn't get hit hard 
) and yelling at her.
She then got down to her DD's level to talk to her and I walked on ahead with another mum who used to be her next-door neighbour. She said something like, "phew, when she loses it, best just keep out the way..."
Everyone eventually calmed down and friend caught up with us and her DD wasn't even crying. (Mine were horrified though.)
WIBU to try to have a word with her today, or should I do as neighbour said, and leave it?
I don't want her to feel I'm judging her but she is clearly struggling and can't go on like this or she's going to end up hurting one of them