I allowed myself to be “stolen” from guy #1 by guy #2. Then I allowed myself to be “stolen” by guy #3 from guy #2, after being with him for about 5 years. Guy #2 didn’t mind stealing me from guy #1, but he didn’t appreciate guy #3 taking me. I realize that the wording is somewhat inappropriate, since stealing is probably not the best term. I willingly left one relationship for another. Still, I was actively pursued while I was in a relationship. I think that my point is while it seems easy to rationalize pursuing a relationship with a person who is already involved, let me echo spoke-'s dad and say “You lose a woman the same way that you get her.”
Depending on the circumstances, it may not be entirely inappropriate to tell her how you feel. I don’t think that you should pursue her beyond that. If things aren’t going well in her relationship, jumping from the current relationship to one with you won’t fix everything.