"Evicted" only applies to a formal eviction proceeding. One thing she could do is say "I am not moving, and will require you use legal process and prove I am in violation of a lease term and obtain an eviction order. I will also not be moving my stuff, so you will be required to hire a state certified eviction mover at your expense if you are successful in obtaining an eviction order.". The lease is almost certainly joint and several, which gives the landlord a claim against all parties for the entire rent (he seems to be ignoring this in his letter), and gives her a claim against the other parties if she pays the full rent. Under joint and several, the landlord probably has no authority to release only some of the parties from their obligation, as it impacts the other parties to the contract.
Simple "I will not move until ordered to do so by a court of competent jurisdiction, and will exhaust all legal processes to resist any attempt to unlawfully pressure me to move from the apartment, since I am in full compliance with lease terms." I will also explain to the court that your suggesting the other parties move is the reason I am on the hook for the entire lease, and will ask the court to void my responsibility triggered by your actions.
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From the landlord:
"Obviously it would be much easier for the others to stay and just fill one room (and I'm confident—were this to happen—that the remaining housemates will release Leyla from any further responsibility under the lease) and that's why I proposed what I did."
More ignorance from the landlord. Under joint & several, a subset of the responsible parties cannot release a party on the lease from her obligation. Only the landlord can do that. Note the landlord was not offering to release her from the lease obligation, but assumed someone else would pay making the issue a moot point.
If you think it is tough to evict someone, try evicting someone and keeping them on the hook as a guarantor of the rent.
The landlord should have paid for an office consult with Langer.