it will depend on the kind of relationship you have with the lady. If your freindship was affecting her relationship it is possible she will choose to save her relationship the her guy. Also, if you wanted to be in a relationship with her despite the fact that you knew she was in a relationship can also result in that.
Another instance is where the guy realize the lady spend more time with you than with him, he will do anything to protect his love. I will advice you stay away from the lady if you want her happiness until she open up to you