Girlfriendd.

Okay so Im dating this girl named Hailey. <3 She everything i wamt and need but more. When i started going out with her she just had came out of a relationship and everyone said that. I get it, been there done that right. Well, she just found out the guy has lung cancer and i dont know.
Well i feel like maybe just maybe if he comes back she will go to him. Her bestfriend, told my bestfriend that she legits loves me and all but i keep being told she still has feelings for Zane. Should i be worried at all? I mean she tells me i am a perfect boyfriend and she loves me but still.

1) Ever had a girlfriend/boyfriend who was like this or still had a little feelings for their ex?
2)What should i do?
3)Favorite song?