ANSWER:

This is not kufr but definitely sinful and you will gain multiple sins continuously for encouraging him and making him continue in such a sin. 

Is saying to a singer that his music is cool and that he should continue (because you love it but you know that it’s haram and bad) regarded as Kufr ?

Sins, Sports, Entertainment and Music | August 12, 2022 4:59 am
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There is a boy music band called “BTS” and majority of their fans are girls (also muslim girls) so the muslim girls keep calling them cute/handsome and start comparing other muslim boys with them. Shouldn’t girls also lower their gaze? Please clarify