Freunde in der Not gehen hundert auf ein Lot

Etymology
lit. 'friends in need are a hundred in a lot', meaning that a friend in need weighs 1% of a (English “”), which is a historic measure of weight equivalent to half an ounce. One could thus loosely translate: “Friends in need are weighed in milligrams.” There are two explanations: either that one’s friends become “lightweight”, i.e. unreliable, when one is in need – or, less likely, that true friends in need are so rare that they have to be weighed in infinitesimal quantities.

Proverb

 * 1) a friend in need is a friend indeed