My girlfriend hasn't told anyone she is in a relationship. And this guy friend flirts with her,she ignores.

The thing is, this guy is her batch mate from uni.he doesn't know she is in a relationship. so thinks she is single. my gf doesn't wanna tell either. even last time through sympathy and all he wanted to get her attention and started flirting. my gf denies. but once agreed with me, stopped hangging out with him, cos even while in study groups, he just wastes time and talk stories. so she just says hi bye only. then again after 5months, she let him be in her study group, cos she wants to help him as well, she said, it's true he wants to do well in studies. i'm ok with that. but he again started flirting, as in email her telling how he feels about other women, how he wants to kiss and hold other women. and my gf refuse to share these things with me. she says it's personal to him, and he only told her, so doesn't wanna betray him. but she is in a way betraying me.though she says she only listen and doesn't flirt back. i dont' want her to be a sexual satisfaction to any guy even by mind.My girlfriend hasn't told anyone she is in a relationship. And this guy friend flirts with her,she ignores.
I understand, tell her, that your dealing with the fact that she is cheating the relationship, she isn't CHEATING, but she is making the relationship something thats on the back-burner and is treating thusly. NOT FAIR!!!! She should be honest to him, her lack of honest DOESN'T give her ANY RIGHTS to be his friends. I'm calling Bull Sh!t, no offense buddy, but her doing this is uncompassinate towards you because the relationship she has with him is based on an F'IN LIE!!! Tell her that, that her friendship isn't a friendship because she's lieing to him about her truth, that she is with someone. So her talking all this stuff about not tell you, is bull, because thats one lie, to make another lie (technically not telling the FULL truth is a LIE) so two wrongs don't make a right, if she was honest she wouldn't have to lie at all. So tell her that she isn't being very compassinate to ether, she's betraing both of you, AND herself, not fair, not fair..... lets just say, if they have a true, healthy, deep friendship, why does she have to lie about being with you? Because secerts don't make friends, she's deseving him, OR the sad part, deseving you, you figure it out....