I vote cheating. Especially if it was a long term/serious relationship (like if they were married, engaged, or even just living together), and if he cheated with someone close to her like her best friend or her sister.

Thanks to Cat for my rockin' avatar!
++++
(About Lois & Clark)
Perry: Son, you just hit the bulls eye. It's like we're supporting characters in some TV show and it's only about them.
Jimmy: Yeah! It's like all we do is advance their plots.
Perry: To tell you the truth, I'm sick of it.
Jimmy: Man, me too!