The opposite of a fair-weather friend, a foul-weather friend only seeks you out if they have a problem, need a shoulder to cry on, a ride to town or someone to watch their dog, but otherwise they act as if they don't even know you. They're only your chum when they're glum.
A foul-weather friend will never call you to go out with other friends, go to a show or just go hang out, but the minute trouble comes, your foul-weather friend knows you are good old reliable you, the one who will listen when no one else will, and they act like you're their only friend. Don't expect the same in return either. When you've got a problem, they'll usually tell you not to bring them down.