Why don't you just break up with her and let her do her thing at school. She's not going to stop hanging out with him and she's very much trying to make this your issue by telling you that you don't trust her when she's putting him/them as a priority over you.
She doesn't value you or the relationship right now and maybe its time for you to let her go, grieve for a bit and then find someone who is more compatible to you?????
I don't know what you could say to her that will make her change her priorities. You may try asking her: " how would you feel if I was spending all of my free time with another woman? "
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion