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.