Someone who treats you only as a "friend" when it is convenient for them.
The type of friend who only calls when they need (money, help, support, advice, etc.) but are never there to help or give it back to you.
They are the type of friend who will want to hang out with you when they are bored and have nothing else to do and have nobody else to hang out with. But, as soon as another opportunity arrives they will leave and not talk to you until they need you again.
Typically a very self-centered type of person who only truly cares about themselves.
When the person treating you as an Friend Of Convenience - FOC needs something:
Amanda: "Hey Dave, I'm having problems with my family, can I come over and hang out for a while?"
Dave: "Sure, I'm always here for you."
When you ask the person treating you like an "FOC" for the same thing in return, but they are feeling good about themselves already and don't "need" you:
Dave: "Hey Amanda, I really had a bad day, my girlfriend left me and I got fired can I come hang-out?"
Amanda: "Um... yeah not right now I'm kind of busy.. I'll call you later" (Actually she was doing nothing but a crossword puzzle, and never calls you back)
When you know you're friend is treating you as an "FOC"
"Hey Dave, what's up with Amanda? She never seems to hang out unless she's having problems or needs something from you. "
"Yeah i know, I'm just her FOC, and I'm pretty tired of being used by her!"