Why the answer is 'fair weather friend' because I think it sounds like she...?

[Luby]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #710 "Friends", question 9

Martha is a .......... She is never there when I have a problem. If I call her to talk about my problems, she tells me that she is busy and hangs up quickly.

(a) friends
(b) fair weather friend
(c) friendship
(d) friendliness

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #710 "Friends", answer 9

Martha is a fair weather friend. She is never there when I have a problem. If I call her to talk about my problems, she tells me that she is busy and hangs up quickly.

Correct answer: (b) fair weather friend
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I chose 'fair weather friend' which is correct,but still a got a question.Why the answer is 'fair weather friend' because I think it sounds like she is good,because fair weather is like good by me.But the following sentence tells that Martha don't care about her friend in her sorrows,so how she could be a 'fair weather friend'

[Foah]

Luby, you can find the explanation for the term fair weather friend from the following explanation at the test.

Fair weather friend hence, means that Martha only likes to be with you when everything is fine and you don´t have a problem, i.e the weather is fine at yours but leaves you as soon as the weather changes, in order to go to another sunny place. Not caring about you. No pleasant behave, is it?
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"Ho ho!" said the clown