It sounds like she's ready to move along (showing interest in another man), but you're not. I don't know your gf, but if she's at the point of already flirting, heck, showing affection to someone else, that's a sign she's not willing to commit here. You also mentioned that things started to drift around the times she had feelings for him, another red flag.

My heart aches for you. It's up to you what you do. Think about how much time and energy you want to put into this relationship. Is she going to continue putting you through this? Is there a chance this behavior is a result of her mental health issues? Is she doing this without any regard to your feelings? Things to think about...