Should I take a chance to be his girlfriend or be a Good friend?

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My girl friend and her boyfriend is in a long distance relationship. I'm much closer to the guy than to my girl friend. we're like sisters. we shares all her bad and good doings. since she's in a long distance relationship we flirts a lot with other boys but she said she loves her boyfriend. And Her boyfriend is always asking me to tell the truth about her girlfriend. The guy is also close to my heart too. I like the guy but I can't because of my girl friend. She trust me a lot. same as the guy he trust me too. the guy is holding on even if he's hurting himself all the time. Sometimes he shows affection to me. And my girl friend knows that. She's just holding on since the guy is holding on. What should I do? Both of them are just waiting to each other to end their relationship even if they're inlove with each other.