fair-weather friends 不能共患难的朋友；酒肉朋友

【例句】

I thought he was my best friend, but he turned out to be just a fair-weather friend.
我以为他是我最好的朋友，但事实证明他是个靠不住的朋友。

A fair-weather friend will only stick around when things are going well for you.
酒肉朋友只有在你一切顺利的时候待在你身边。