Ask a landlord-tenant lawyer and get answers ASAP
Hello and welcome! My name is ***** ***** I will try my level best to help with your situation or get you to someone who can.
If A is smoking marijuana in B's house, then only A is breaking the law. B is not, and is not under any legal obligation to stop any criminal offense unless they are law enforcement.
As for whether they can be evicted, if the lease has a clause that prohibits illegal activity on the premises, and B is allowing A to smoke, then they are allowing the illegal activity and that could be grounds for the landlord to terminate the lease and evict.