fair-weather friend 酒肉朋友
fair weather 晴朗天气，好天气
He's just a fair-weather friend.
他只是一个酒肉朋友而已。

A friend in need is a friend indeed.
患难见真情。

lady friend 小三
mistress 情妇
gentleman friend 情夫
Actually, Olivia is not his wife. She's his lady friend.
其实，Olivia 不是他老婆。她只是他的情妇而已。

出轨
two-timing
cheating
He is two-timing his wife with Olivia.
他背着他老婆出轨了Olivia。

third wheel 电灯泡
I don't want to be the third wheel.
我不想做你们的电灯泡。