Most Helpful Guy
If you want an exclusive relationship with her, then, yes, it could be disastrous if you were to make a move, especially if she has feelings for him.
Keep in mind the circumstances in which she is coming on to you under. Her guy requires distance between them. So, from this, I assume she does not feel the relationship with her guy will last which consequently has significantly lowered self esteem and self worth. Thus, she is emotional, vulnerable and perhaps hitting on you, etc., to alleviate her pain.
That, or she may simply want to cheat or perhaps is the type of person who is dependent and feel the need to always be with someone or in a relationship.