What is she thinking?

[Novelist]

I have a friend and she has a boyfriend, but she is single.  For at least 6 or 7 years, she has been committed to this relationship, however, she is going through a dilemma with liking someone else.  Why is she feeling this way about this guy who mutally knows her boyfriend as well.  I have met him as well and he is nice, but I believe there are some struggles with his relationship with God pertaining to his lifestyle.  What I mean by this is that he has been involved in homosexual relationships and possibly could be bisexual, I am not sure, but he has not been dating women lately, I know for sure.  Moreover, she has feelings for him and I have told her that it is wrong to allow those feelings about him to get out of control.  Throughout life, people will meet others and become friends or even more, it can happen, but she is in a relationship and she finds herself thinking about this guy, talking to him on the phone, and feeling this romantic pleasure.  I know this is not right and she knows too, but she would say that she cannot help it.  There were times when she tried to avoid talking to him, she was even jealous when he brought a friend to church and I found that crazy! How could she be jealous and she has the love of her life?  

Before my eyes, I have witnessed the way she feels because I can see it on her face.  She is mesmerized by this young man and I am wondering if he feels the same about her too.  It is complicated because of his battles with his lifestyle and of course, she needs to build a closer relationship with God as well.  On many occasions, we have talked about this and I would tell her that women as myself would be marveled to be in love and have a long term relationship and how she has feelings for someone, knowing that it cannot go further than friendship.  Is this a phase or what?  Sometimes, we can like someone, but if you are in a committed relationship, you do not want to jeopardize your relationship over something that is not real in your heart.  I wish she would realize that she has a good man and stop feeding her heart with other thoughts of this other young man because she has strong feelings for him and it is crazy.  Sometimes, I wonder if she would ever get over it.  As a close friend, I tell her the truth and basically I tell her that these feelings need to go and she has to either let it go or let him go, but she does not want to do that, which I know it is hard, but how can they continue to be friends knowing that her feelings or even his feelings will be covered by playful flirting, which she claims is harmless, but I do not agree.  What should she do in this situation and is this worth going through as far as her feelings?