Не тот друг, кто на пиру гуляет, а тот, кто в беде помогает

See Друзья познаются в беде (Д)

Cf: Feasting makes no friendship (Br.). A friend in need is a friend indeed (Am., Br.). Friends are made in wine and proved in tears (Br.). Prosperity makes friends, and adversity tries them (Br.). Prosperity makes friends; adversity tries them (Am.)