fair-weather fan
nounUK /ˌfeə ˈweðə fæn/
US /ˌfer ˈweðər fæn/
A fair-weather fan is someone who only cheers for a team when it is winning.
He stopped watching the games as soon as the team started losing—what a fair-weather fan.
OTHER MEANINGS
2. Someone who supports a person or cause only while it is popular or successful.
She was a fair-weather fan of the charity, donating only when it got media attention.
3. A supporter who abandons a team during a losing streak and returns when it wins again.
The stadium emptied fast, leaving only the true fans and none of the fair-weather fans.
4. A person whose enthusiasm for anything fades as soon as it becomes difficult.
He's a fair-weather fan of fitness—he quits every gym by February.