How do I deal with her?

Well first off kudos to you for being such a great friend! The fact that you have strong feelings for her is going to make this situation a whole lot harder, but I would also say the fact that you care about her very much since you've known her for 6 years says a lot in itself. Personally if I were you I would stand strong next to her and try and get her through her situation, guide her, show her that this situation with this other dude is not a good one. Do all that you can, but don't push her to the point that she gets annoyed with you either. I can see where your confused, it's a fine line. Do you step in and be a friend? Or do you walk away?....I would just say do all that you can to steer her away, if she does, then let her sort some things out on her own. If she doesn't and stays with this dude, then walk away for awhile. If she writes or calls etc then sure be there for her, but let her know you have a life too and you have done all that you can to support her, but you can't wait around forever ya know? Good Luck!