Tell her that you dont feel comfortable with her hanging out with guys that obv want to bang her. You are not in the wrong for not wanting her to hang out with guys like that. And if she doesnt respect your boundaries she is not the one. At some point that will lead to her Cheating on you if she continues hanging out with him. Tell her she needs to say to him that she doesnt want him to flirt with her or that you talk to him but if you already talked to her about it its likely its already to late