What should someone do if they're in love with a friend's significant other?

Feelings in and of themselves are not good or bad. It's not against the law to feel like hitting your boss; it's against the law to hit him or her. Your friend would be much better off not allowing himself to label his attraction "bad" and putting his focus on not acting on his feelings. The way he needs to do this is to make it his priority to [i]never[/i] be alone with her. It would be best if he limited or avoided all situations that encourage flirting or losing one's inhibitions, such as drinking a lot of alcohol (a lot means whatever amount tends to lower his inhibitions). He needs to carefully monitor his actions, not his feelings; if he is a good friend and an honorable person, his feelings should be strong motivation for that monitoring, in other words, the stronger the feelings, the more careful the monitoring. And, needless to say, none of this should be discussed with her or anyone who knows her or could possibly know her or ever talk to her.