Yes, it is ridiculous for your friend to completely change his preferences based on something that is not at all related to music. Doing drugs is bad, but I don't think that makes me dislike The Beatles' music at all. If I found out that Bono was gay, it wouldn't suddenly make me hate his music.
I think you're lumping all Christians together, which is slightly unfair. Yes, there are those radical people who immediately cut off all contact with anything they find impure, but there's a fair lot of us that just love all people.
I know God and Jesus love Freddy Mercury. Sure, they may not have approved about what he did in his life, but hey, there's still time for him to learn the truth up in heaven.
Oh, and I don't think it's really fair to lump homosexuality with race. You can't choose to not be black. (excluding Michael Jackson, of course)
Yes, homosexuality is something you're born with, but you never have to act on it. I know this might be blasphemy to some of you, but in my mind it's no different than being born with a disposition to become an alcoholic. Sure, you COULD end up really bad off, but you never have to take that first drink. It's just another trial we're given on this earth.
Oh, and don't say that I don't know anything about homosexuality. My dad just got out of prison a couple of months ago. He was convicted for hiring hit men to murder my mom, because he decided he wanted to be gay. I kind of have a little bit of experience in the matter.
Anyway,
tl;dr, no it is not reasonable for your friend to act that way. Learning he's gay in no way changed anything about the music he once enjoyed. He's just generalizing that everything gay = bad.