A friend is known in time of need.
More French Proverbs:
You know a person by his nickname.French
Twixt the cup and the lip there's many a slip.
French
It is only idle people who can find time for everything.
French
A good fox does not eat his neighbor's chickens.
French
A fool's voice is known by a multitude of words.
French
He who has a stepmother has the devil at his hearth.
French