It's not a nuisance, but since the neighbors don't want the correspondence, she should stop. It is unlikely she'll be evicted because it will look like the landlord is retaliating for the habitability complaint, which is illegal.
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Why would she think she had to send a letter to other tenants? The other attorney is right about not sending the neighbors anything. Honestly, this case sounds a little strange. Why would neighbors threaten to sue her over getting something in the mail once? I think there is more to this story than is contained in your question. If this is something that seriously needs to be addressed you can call my office for a consultation.
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