I am sympathetic. I was on the receiving end of verbal and physical abuse throughout school.
But ...
There was a report on the radio this morning, which I can't confirm, that the mother may have grabbed the wrong kid. That this was not the one who tormented her child. That's one of the problems with vigilante justice.
Even children have to be considered innocent until proven guilty.
Moreover, she just emphasized that if you are bigger you get to assault someone.
If the school was doing nothing, siding with the bully as authorities often do*, then she needed to use the justice system. Take her child out of school until they provide a safe environment. Use social media.
There was a case in Oregon some years ago, a man on trial for raping a child, the child's mother brought a gun and shot him during the trial. I'm going by memory so I may be wrong in some details. And the same argument. Yes, everyone was sympathetic but is vigilante justice the answer? He had not been convicted.
** One school district recently gave the students literature on bullying in which they were asked to figure out what they had done to cause the bullying, and not to tell because no one likes a crybaby. They withdrew it after parents screamed bloody murder.
If I were on the jury I might have to convict but I'd say probation.