Future advice for your friend, don't get in the way of his brother - yes his brother had something illegal which he should have, but for future reference it may not be worth letting him snap over.
Getting in an argument with a person with a mental illness isn't the wisest of things, even if they're nice - my brother suffered childhood trauma on the account of an abusive teacher, so when he loses his temper he can't control himself and it doesn't help that he has never had a particularly long temper. I almost got in a fight with him myself, luckily for me I'm a martial artist and put him off - but he's still a lovely bloke, just don't get on the wrong side of him, sometimes it's inevitable that you will - but handle yourself differently.
Of course on the other note, 'I have mental illness' still isn't an excuse to kick the crap out of somebody, you can't control your flips but you can control your temper, which trigger the flips - by the time you're an adult you should be mature enough to understand that you need to deal with your problem. Of course if it is something you can't control, he needs medication or treatment. My uncle has schizophrenia (after many years of drug abuse) he has to take medication, why? Well when he's set loose he can be violent, attacked my Nan once and has been subject to arresting - luckily my Nan didn't press charges, instead just got a restraining order put on him - he should have taken his medication.
The fact his brother does drugs (it was a bong he was trying to get?) and has a mental illness, I really think should be considered and brought forth to the police, because A: Mental Illness + Drugs = bad idea B: Drugs are illegal. So I think it would be wise for the coppers to take him into rehab and perhaps (if the brother hasn't already) advise on a psychiatrist or mental evaluation - if he isn't mentally fit for society, well do what they've done with my uncle on different occasions, take him to a mental asylum until he is mentally fit enough to walk among the sane.
"Experience never provides its judgments with true or strict universality; but only (through induction) with assumed and comparative universality." - Immanuel Kant
