Animal cruelty. Should I report?

[DeeSpark]

Ok so I have a group of 'friends' at school. Not sure about being friends with all of them, to be honest I don't think I'm sitting with the right group at school, much happier with that OTHER group, but that's besides the point.

Now, these people, while not very compassionate about animals in the same way as me, aren't into animal cruelty. There's one guy in particular who plays an important role in this situation. He's always cared about his guinea pigs. Not the best owner, but he's never really liked it when his brothers drunken idiot friends try to hurt them.
We just got back from an easter break. All from word of mouth, I now know that his older brother bashed a dog for biting/killing (not quite clear on which) and cut in half and stabbed both his guinea pigs. He's brother, I know is the biggest idiot, but for some reason everyone thinks he great. After all, what's not to love about a guy who abuses animals, takes advantage of young drunk girls, and has had several brushes with the law?

Anyway, I'm unsure of what to do. Animal abuse isn't really his normal crap he does. Both were one off occassions, the dog got bashed for biting the cat and they didn't want the guinea pigs, and won't be getting more. Not at all saying that is an excuse, I think it's disgusting. But I don't really have any proof. I mean, like anybodys going to say anything if they know about it. Reporting them is fine but if there's nothing but my words to prove that he actually did anything. What can I do? Should I atleast email the RSPCA and ask if there's anything that could be done with no evidence?  

[JosephineReally]

Does he still have the dog? If he still does then I would contact some one and either give an anonymous tip or see what you can do to insure that it doesn't happen again for sure. From the way he sounds to me.. it might not be a one time thing.  

[DeeSpark]

No it was a random dog that unluckily chose to go for his cat...

First time I've heard of him doing any animal abuse, but his already been in a lot of trouble lately with the police for drinking, drugs, breaking into places etc. No doubt he'll end up in jail soon anyway....  

[pompoennetje]

I would send them an email or a call. Maybe explain them the story? So they can judge them self.