The landlord offered to let the gun owner out of the lease without the normal “roommate change fee” and had secured a commitment from the other women to fill her spot. If they couldn’t work out their differences and she didn’t want to move the only other option would be that the other roommates could move out and she would have to hope they’d voluntarily pay two rents until she filled the apartment. As @CIEE83 notes, due to joint and several liability if they refused to pay the landlord could go after her for the entire rent and she would be responsible for collecting from them somehow. Of course she can refuse to to move. I see no indication the landlord has made any move to evict her.
Thinking you are right, ethical, morally upstanding or fill in the blank and want to do something is different than following the letter of the law. That’s the issue IMO. Just because you don’t like guns doesn’t mean you can break into someone’s room/apartment and take matters into your own hands.
My monies on her. It has already come out in various discussions that there was no restriction on her having a gun in that apartment, i.e. there was nothing in the lease. She also graciously said that she wouldn’t press charges against the roommates trespass ( though has reserved that right). I have rented in MA ( many years ago) and I have never, ever heard of a gun clause. Ever. And I have also rented in a multiple roommate situation in MA similar to this where someone didn’t pay rent. The landlord lost the money not the individuals. Each and every time. Students usually sign separate leases. I knew many students who rented and no one ever went into someone’s room and then called the police based on what they found.
Someone mentioned that there are signs in AZ stating no guns in retail store, again state laws vary. Because you see something in one state does not mean it holds in another. I’m going to step out of this as it leads too close to argumentation. The points have already been made. For some, “the end justifies the means” seems to be their matter of thinking. Seems like the case will play out on its own.