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Someone who remains a friend only when things are going well but abandons others during times of trouble or difficulty. I thought Allison and I had a strong friendship, but I learned she was just another fair-weather friend when she stopped talking to me after my divorce.
Fig. someone who is your friend only when things are pleasant or going well for you. Bill stayed for lunch but he wouldn't help me with the yard work. He's just a fair-weather friend. A fair-weather friend isn't much help in an emergency.
A person who is dependable in good times but is not in times of trouble. For example, You can't rely on Sarah-she's strictly a fair-weather friend. This expression likens fair weather to good times. [Early 1700s]
a fair-weather friendsomeone who cannot be relied on in a crisis.
1998 Spectator The Americans gave up supplying gold on demand to other countries' central banks at £35 an ounce…when their fair-weather friends from London threatened to turn up and clean them out.