Oh, such a difficult question, Jently! Decisions, decisions. I would probably be secretly really happy about it and start going back and forth in my mind on whether it's possible for the person to leave their partner. Then reality would set in.. and it would definitely weigh on my conscience--upsetting the balance of their already established relationship is bad. A love triangle could be dangerous territory ending in ruined lives, or even murder! If I liked the partner so much, then I would just settle for being friends since he/she is already taken. I would want for their relationship to have honesty and closure before the partner started another one.
Continuing your question, what would you do if your friend's partner, who you really fancied, decided to leave their partner for you, and you know your friend will be in world of hurt from the decision? The partner feels that you are truly "the one", and in their eyes, their heart and soul already belong to you.