oomis: “the person who called the school (and so quickly) was guilty of L”H”
Actually this would be a classic case of to’eles and 100% permitted if done properly. If I had a teenage daughter who was hanging out with unsavory boys I would certainly want to know about it in time to deal with the issue and not find out only after it is too late. I find it hard to believe any parent would feel differently. It also sounds like the school dealt with the matter very responsibly, not threatening to expel the student, just informing the mother that “they thought she needed to know”.
However I do think the school should have been left out of the picture, the classmate should have asked her mother to call your friend’s mother directly.