What to tell her to do?

my cuzin is going out with this boy for like 7 months or so. They are having problems because he is hanging out with this girl that he met in his school after they started going out. They are supposedly good friends and my cuzin knows that they are just friends and she didnt catch them doing anything together. But she says that there are limits to that and that she feels they hang around 2getha too much. I suggested a compromise but she doesnt want to compromise with him. She sed that she also stopped talking to people for him like her best friend stephanie. But he says that the reason why he told her to stop talking to stephany was because according to him she was trying to break them up and was two faced to her. he sed that stephany stopped talking to her when she was going out with this boy and when stephany and her boyfriend broke up the first person she went to was my cuzin crying. So that was sort of different. What should I tell them each to do?:>