まさかの時の友は真の友。

English Translation

A friend in need is a friend indeed.

Is this a common saying?

@ericaw
The English sentence is a proverb, but there is no equivalent one in Japanese. The Japanese sentence is one of the common translations I saw online, but I have never seen anyone saying it in Japanese.

If you prefer a typical and unique expression in Japanese, I would recommend 朋友は六親に叶う (ほうゆうはろくしんにかなう). 朋友 is a best friend. 六親 refers to next of kin: father, mother, elder brother, younger brother, wife and child. The proverb means “one best friend is equivalent to six members of your next of kin, so you always need to cherish the friendship”. Your next of kin is supposed to be on your side when you need help. Equating a best friend to your next of kin can be interpreted as “A friend in need is a friend indeed”.