The part about asking him seems fair. I am drafting a letter regarding this and other issues.
How would flagrant and repeated drug use in common areas of the apartment building not be a habitability issue? It's not people doing drugs in their apartment and it's not "people." It's tenants and their guests in a common, semi-public area.
There are other habitability issues under Civil Code §1941.1, but I am particularly interested in how to tackle this one.

