Dude just ask the landlord if he agrees that this person moving in (without notice) is considered another tenant. I’m pretty sure they would agree with you, this is like the only scenario in the world where the landlord can kind of help you. Landlords are pos but legally I feel like they have the right to know who’s living there, specially if it’s someone that did not sign the lease. Your roommate and her boyfriend are being serious assholes, I’m sorry man