A fair-weather friend 酒肉朋友

人们常说：患难见真情。意思是只有在困难时期陪伴、帮助你的人才是对你真心真意的朋友或亲人。相反，在 “好天气 fair weather” 时相伴，但一有事情就弃人于不顾的朋友在英语里叫做 “a fair-weather friend”，也就是汉语里说的 “酒肉朋友”，不能与你患难与共的朋友。

例句

I thought Mark would always be by my side, but I realised he was really just a fair-weather friend. When I needed his support, he wasn’t there.
我以为马克会一直陪在我身旁，但我意识到他其实只是个酒肉朋友。在我需要他的帮助时，他却不见了踪影。

When it becomes clear that someone is a fair-weather friend, you should walk away from the friendship. They will only disappoint you when you need them.
当你发现某人肯定不会与你共患难的时候，就应该放弃这段友谊。因为他们只会在你需要帮助的时候让你失望。 

I went through a difficult time last year. It made me realise who my real friends were and who were only fair-weather friends.
去年我经历了一段很困难的时期。这让我意识到究竟谁才是我真正的朋友，而谁却只是酒肉朋友。