It might really upset my friend when I tell her that her favorite artist is homophobic.
For what reason do you want to tell her that the artist is homophobic?
It makes me really happy when I break her heart
You're intentionally lying to your friend for your pleasure and it's wrong. She's going to know that you were lying about her favorite artist and you will possibly destroy your friendship.
Who cares, it's not like she can do something to hurt me.
I can't wrap my brain around your nonsensical desire to hurt and upset someone who you claim to be your friend. You shouldn't want to upset her, but at this point I think you need to assess as to whether you truly think of her as a friend. Rather than hurt her you should probably just end the friendship.