fair-weather friend

1. A friend who supports others only when it is easy and convenient to do so: “I thought Gene would always stick by me, but when I got into trouble, he turned out to be a fair-weather friend.”

Idioms and Phrases

A person who is dependable in good times but is not in times of trouble. For example, You can't rely on Sarah—she's strictly a fair-weather friend. This expression likens fair weather to good times. [Early 1700s]