Landlords powers are limited when it comes to the behavior of their tenants. I had a similar situation arise and it was the blame game. One would complain the other was harassing them, the other tenant claimed it was the other way around. I suggest you do a google search for California LL Tenant, substantial violation etc... there is likely a statute that would give the LL just cause to terminate a tenancy for assulting another tenant. The problem is the blame game. The LL will need proof and this means that a police report and officer findings will play a big role. As the others have suggest, call the cops and let them investigate. After a few calls and some history on this bad guy, the LL will have cause to evict. Problem is the LL may not be able to get police reports, especially if these are minors.
I would recommend that the victim to the assult obtain a copy of report and submit to LL.
If you get too involved in it, you may find yourself looking for another place to live so tread lightly.