What To Do!?

My mind is totally in a mess now while my heart is not moving as usual.
During orientation I met a girl whom smile was totally pretty for me. We got to know each other more in the past week and got pretty close. We sms each other very frequently and talk on the phone once in awhile. In our conversation she flirts with me a lot and I always reply with a flirt too, to many it would seem that things are going smoothly for me and I would once again be attached but things are far more complicated than it seems.
A few days back she told me she already have a boyfriend but he is very far from her, Singapore to be exact, and they have too many fights which is making her tired. From this some may think that my chance has come but my personal thinking is that I would not want to break up someone's relationship or be the reason of the breakup. Even if they broke up and we got together it would most likely mean I would be the spare tire.
Just recently she had a fight with her boyfriend and she was really down, she wanted to meet up with me. She was feeling down and sad, her mind was spinning around too I think. I tried my best to care for her but should I even get myself involved in this matter? I think I have no say in it and should keep my ass out of it but why did I go and do something as to care for her? A good thing is that I couldn't meet her so things won't get more complicated.
It has been quite some time since I was involve in relationship matters but meeting something this complicated is a 1st for me. Even now I feel that I am just someone that is able to give her what she seeks from her boyfriend which indirectly means spare tire again. We actually agreed to keep our relationship as friends but than with the flirting in the messages or phone calls of care I think the relationship is kind of over the boundaries of friends.
How should I tell her that since she already has a boyfriend we should keep a distance and maintain as friends? How to tell that I feel like a spare tire and I don't want to be one? How should one not hurt her feelings when telling all this and not hurt the friendship? What should one do now?
The main reason I'm writing all this is to seek advise from the outside world but I doubt anyone would reply to it. Matters of the heart is too difficult for me to settle although my heart is kind of dead long time ago.