Two vivid physical images: a broken tooth that fails when you need to bite, a dislocated foot that fails when you need to walk. Both are painful and useless at the moment of greatest need. The unfaithful person is exactly like this — worst when needed most.
Memory phrase: Trust only the truly faithful.
Talk about it: What makes someone truly trustworthy to have around in hard times?
Prayer: God, help me be the kind of friend who never lets people down.