Go through the lease. Find out where it says that having a legally owned firearm is grounds for eviction. If it isn’t there, get a lawyer and sue him for harassment and intimidation and force him to evict her.
There are no federal constitutional violations because there is no state action, meaning no action by a government agency or actor. Unless there is some other available remedy in state law, the best option would be to sue for breach of the lease contract. Of course, there may be provisions in the lease that permit termination due to the reaction of the roommates, and the landowner could decline to renew her lease. Alternatively, the gun owner might arrange to settle the controversy by storing her firearm off the premises.
Bingo!!