Someone who is only with someone else for their money, popularity, or good reputation. They dump the person as soon as things start looking bad for the person.
Fair Weather Boyfriend/Girlfriend:
A- "Hey, have you seen Lauren's new boyfriend?"
B- "Yeah, I've seen him with some other girls before. He's a total fair weather boyfriend."
A-"Zeke, where's Amy? Did she dump you?"
B- "Yeah, I'm poorer than a hobo now. Amy found out and broke up with me, what a fair weather girlfriend."