A fair-weather friend consists of a pair of words in different languages that are cognates, but with multiple meanings, some of which transfer and hence are true friends, and some of which don't and therefore are false friends.
English "become" and German "bekommen are fair-weather friends because although cognates, and having generally similar meaning (i.e. to become), German "bekommen" can have the meaning "receive"/"get", which English "become" can't.
Many words that are labelled as false friends are actually fair-weather friends.
Following the tradition of false friend and true friend expressed in French as faux ami and vrai ami, fair-weather friend is ami des beaux jours (m); literally "friend of the beautiful days".