This is not an AIBU, no need to shout. It's a genuine question.
Ex-H and I have 50:50 and a good parenting relationship. All this happened to Ex-H so I am getting details via him.
The background: DD 8yo goes to a very small village school, her year has only 11 children and they are in a class with the year below which has 8 children. Girl A in DD's class has a very dominant personality and a strong friendship with Girl B. Girls C and D are on the fringes of that group. My DD is friends with girl E and some of the younger girls. Girls C and D sometimes also play with my DD.
Girl A has been involved in some controlling and bullying behavior but not to my DD. Girl C has born the brunt of this and is frequently in tears at school (also at pick up). Her parents have approached the school about the bullying - obviously I don't know any more details than that. Girls B and D may also be getting bullied.
My DD had a sleepover with Girls C and D. During the sleepover Girl D made fun of Girl C for her appearance, said she was ugly, compared one of her features to an animal and told her she was their servant. Ex-H overheard all this. DD put a stop to it - I am very proud of her - so Ex-H did not have to intervene directly. Girl D stopped after my DD intervened.
Should Ex-H tell the parents of Girl C and Girl D what happened? Girl D's parents are likely to be very sensitive to any criticism of their daughter. Rather than considering the possibility that their child is misbehaving they may be offended and refuse further play dates (IMO perhaps not a great loss as Girl D is not behaving very nicely but as the school is so small there are very few opportunities for making friendships - three of DD's best friends left last year). I think Ex-H should definitely tell Girl C's parents.