You use fair-weather to refer to someone who offers help to someone, or who takes part in a particular activity, only when it is easy or pleasant for them to do so.
...a fair-weather friend.
ADJ : ADJ n [ disapproval ]
FAIR-WEATHER
Meaning of FAIR-WEATHER in English
Collins COBUILD Advanced Learner's English Dictionary. Английский словарь Коллинз COBUILD для изучающих язык на продвинутом уровне. 2006