It's bad to sleep with your friend's significant other.
Charles, who has a friend that is dating a mutual friend, broke up with his girlfriend and feels lonely.
Charles wants to feel less loney.
Charles calls his friend and asks to hang out with both him and his girlfriend to cheer him up.
Charles calls his friend's girlfriend, persuades her to spend a night with him, and does so.
Charles' friend is happy for the chance to help Charles feel better and the three of them have fun together.
Both Charles and his friend's girlfriend regret it afterwards and it makes things very awkward and tense between them.