We got different characteristic. They maybe good at home but bully at school, because they can't let go their anger inside their home. And some bully at home that's good at school or outside the home premises.

This guy is in trouble.  Maybe he wants to show a good front to his parents but shows his real side in school?

there's not much you can do with him. it's his behavior and nothing can make him change that except himself. give advice if you can, but to change him? i don't think so...

lots of two faced people in the world and I am sorry to say that your friend is one of them  sooner or later he will learn his lesson and then will change - hopefully!

Tell his mom?  Seriously, if he hurts kids then the higher ups should know!

teenagers typically pressured by peers to act violently. don't you think this could be one of his reason? or maybe he just want to be independent from his parents and this is the only way he could do that.