Someone you use in order to stop boredom because all of your other cool friends are at work, with family, etc.
A Friend Crutch is a person who is usually obnoxious or a tool on certain occasions, but won't make you flip out every minute.
Guy 1: You hanged out with Ryan yesterday?! Dude he is such a tool....
Guy 2: Don't worry he is only a friend crutch, everyone else was busy.