Some people wouldn't consider that a favour at all. That was her own problem, and you only made it worse.
I think that yes, you should forgive her. Find out from another source what became of that romance, and whichever way it has turned out (they're back together, or they've broken up for ever), apologize for getting into her personal business back then.
Don't mention "forgiveness", since from her point of view you were the one who caused her great distress at the time.