Fair weather

[Sonia]

Applied to someone who is present only when things are going well or when they need something, but at the first sign of trouble/hardship, they disappear faster than [Brittney Spears]s career
In sports, the term is often confused with a [bandwagoner], but unlike [bandwagon] fans, a fair weather fan initially has some interest in the team, but they want nothing to do with them when tough times occur. As soon as the team starts to turn things around and starts winning, they act as if they been there through thick and thin.
A fair weather friend is more of an associate/acquaintance. They arent truly your friend, but you talk to them enough to somewhat know them personally. They act nice to you when they need something, but as soon as you need them to return the favor, they are nowhere to be found. A [true friend] is there for you no matter what.
This term can be applied to anything in life, but these are the most common.

[Luis Fok]

a fake friend.
who will be a best friends, but be careful, because they will backstab you quicker than lightning.