Seems to me that if the person knew you were there, even if your partner was lying about how bad your were to gain their sympathies, then it's fair to hate them and despise them for cheating just as much as your boyfriend/girlfriend. It takes two to cheat, and if they know then they're just as guilty.

On the flip-side, the part about not hating the person if they honestly didn't know is also very good. It might be too painful to interact with the person because it brings up all those memories, but hating them isn't fair. They were led on, and feel the exact same as you.