This won't end well for her son, no matter what happens. Whether she approaches the mother, or whether she requests the records, or both, he is still going to be the kid that tattled to mommy. It's already out there that he is a "baby" and a "snitch." It's just going to keep snowballing. Unfortunately, that is how teenagers are.
The issue I have, here, is that everyone automatically believes the mother did it, and is, basically, adopting a "guilty until proven innocent" stand. They are judging the mother (without knowing her) based on the actions of the daughter. Everyone is assuming this woman is some sort of horrible person, might she be? Yes, but she might also be a perfectly normal parent to a perfectly normal teen girl, who has no idea what crap her little princess is spewing at school.
I get everyone's desire to always defend and believe your kids. I get feeling angry because your kid is hurt. As a parent, I would hope that another parent would give me the benefit of the doubt and have enough respect for me, as an adult and parent, to have a civilized discussion. I know on the Dis there is a higher than average percentage of gifted and perfect children, but having been a teenaged girl, and having a teenaged girl, I know how they can be. I would hope that a reasonable parent wouldn't, automatically, assume I was a horrible person based on my teenaged girl, acting like a teenaged girl. Just think how you (general you) would like to be treated in this situation. I would much rather a parent approach me, than make trouble on my job.
There are 3 sides to every story, this one included. What the OP's son says, what the girl says, and the truth, which is probably somewhere in between the 2 stories.