Dude needs therapy. Disclaimer: everything I'm about to say assumes the guy was never actually violent with the dog.... I'm probably going to get a bunch of hate for this, but.... kids can do some dumb shit. I did dumb shit when I was a kid. I never put my peter in a dog's mouth. Hell, I never got sexual with any animal as far as I can remember. But I did weird stuff. I'm grown up now. I'm pretty normal. Honestly, if we weren't so taboo about sex in this country I think most of the weird sexual stuff kids do when their kids wouldn't ever happen. I mean, my parents didn't talk to me much about sex, and never ever talked about masturbation. It would have been great if dad would have said, hey kegor, rubbing one out is very normal so do it as much as you want, just be appropriate about it.
My point is that kids have sexual urges and often have absolutely no outlet for them. They have to keep them secret. It's stuff you don't talk about. Unless you're one of the lucky bastards that's getting laid when you're just 14, most boys are beating off and feeling nothing but shame and taboo. But then the taboo starts to take on a sexual nature of its own. Next thing you know you're looking for stuff that's taboo but still not too far out of line. Like putting your dick in a dog's mouth. Damn, there are gown ups that do that shit.
Honestly, the dude just needs some therapy. Someone to talk to about the weird shit he's done. Someone to help him understand that it's OK to forgive himself - it's not just OK it's pretty fucking necessary to forgive yourself if you're ever going to get past your issues and get on to leading a normal life.
OP tell your friend to get some help. Tell him that just because he did weird shit doesn't mean there's something wrong with him. He's not the first person ever to do strange taboo stuff with his penis and he won't be the last. But he needs to talk to someone about it so that he can address it and then get on with his life. And really, once he gets over it he should leave the past in the past. There's no need to confess all your life's weirdness to someone once you've dealt with your issues.