There is no indication that the lease says “no firearms”. That’s not what’s going on. The landlord is not seeking to evict her.
Basically the landlord is performing an informal mediation between quarreling co-tenants and suggesting that the most sensible solution would be for gun owner to move. But he’s not saying she has to. He is saying that if she stays and snowflakes all move, he’s going to be looking to her for the $6,000 rent each month. And he has every right to do that.
Sounding hopelessly naive here, but what if she offered to take her roomies to the range & teach them how to shoot and lose their fear of firearms?
Isn’t that the most effective way to change a liberal’s attitude about guns?