DUDE, They are right, DROP IT!
If she cheats on her boyfriend now, you think she'll be faithful to you? NO WAY!
I have seen this many times myself, and I was in the same situation you were in. DO NOT FALL FOR THE ACT!
Ok, here is THE answer to ALL your questions about this situation. It's a long read, but it is 100% correct:
http://www.intellectualwhores.com/masterladder.html
Ask anybody, and if they are truthful, they will tell you the ladder theory is 100% true.
Also, if you choose to ignore the ladder theory (NOT recommended), the only way to actually get this girl is to ignore her. Seriously. DON'T cuddle, kiss, call, or fall to her every whim. She will want you more. It will be like trying to kick heroin, but DO IT. You gotta be an a$$hole.
Girls always go for the a$$hole, not the nice cuddley type. Right now, her boyfriend is an a$$hole, and she will always pick him over you no matter what because you are the nice guy. She has you for all her emotional needs, and him for the rip-roaring, animalistic, no holds barred wild and crazy sexual needs. And even if she does pick you, I assure you she will be screwing him every other week or any chance she gets every single time she sees him.
Trust me, I wish to God that someone gave me this advice before college. It would have saved me much heartbreak and wasted hope.
So from brother to brother, you gotta grow a pair, cut off the relationoship, and see what happens.
For more information, go to
www.tuckermax.com.
I know it may seem harsh, and it is, but I was in your situation once. If you need to talk or some advice, just give me a pm dude.