That's a hard one to answer. But, as a woman, I can say that (for me) if I start to have feelings for someone other than who I'm with, it's usually because I've felt an alienation of affection from the one I'm with. Be it emotional, physical, or a lack of common interests. It's easy to start to feel things for another who takes an interest in you, when your partner has forgotten how, or fails to up keep your "needs". I would start by asking your friend if anything has changed in the relationship, and have him ask Her if she it's fulfilled by him. Maybe he could try to "rekindle" what they had by doing the things that he did to win her heart to begin with. People get comfortable after awhile and simply take each other for granted, and often just stop trying to keep the sparks going, and then the flame dies out. Hope this helps.