One of her distant friends asked me for help
Some friend *rolls eyes* if she'd get herself involved with you like she has when she knows her friend is your girlfriend.
Do yourself a favor and distance yourself completely from this so called "friend" of your girlfriend so that you can clear your thoughts and happy pants from her long enough to put your focus back on your girlfriend. Right now you are cheating on her and you can't make a proper decision as to who you want to be with when you're screwing around with one and pretending to be in a true relationship with the other.
if after a good length of time (you decide the limit) you find you are not happy with your current g/f, if you find you are still bickering all the time then it's best you leave her and not keep up a farce of a relationship with her. Don't mistake the habit of having her in your life as love. If you're going to leave her do it because you know being with her is never really going to make you happy... certainly don't do it for the infatuation of a girl that is willing to help you cheat on her friend. Ugh... where are her personal boundaries? Where is her loyalty to her friend? She may be "fun" and you may be lusting after her but she's not good people in general.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion