I am totally against breaking up couples.
So the advice I'm going to give is purely for academic purposes
The biggest thing, is to just not let it effect you. Game on as usual, if she starts talking about her boyfriend, keep eye contact, blank expression (poker face), don't say anything. When she's done change the subject. Kino is important, hi-five her and hold onto her hand afterwards, stare into her eyes, just make sure you come across as confidant.
There are specific routines for boyfriend issues, straw-man technique is one I'm aware of, and I've heard that it works pretty well. But like I said I'm not into breaking up couples.