Like others have said DEAN's Sorry Not Sorry is fantastic. I'm not really into k-pop so I can't suggest any English songs by idols besides maybe Wedding Dress by Taeyang. BUT, The Black Skirts is a Korean indie artist who grew up in America and he's gotten his way into my top ten favorite musicians of all time. If you're trying to get your friends to open up to Korean artists he is without a doubt the way to go, here are two fully English songs by him and two that have a ton or a fair amount of English:
Stand Still (fully in English)
International Love Song (fully in English)
Love Shine (3/4 in English)
Hollywood (primarily in Korean, but has a large verse in English + the sound of the song is very American style, so is the music vid)