Should I hire a lawyer if Unum is demanding overpayment from SSD when we didn't want SSD in the first place?

My mother has been deemed permantly disabled due to her health she has Unum thru her employeer she was on short term then went to long term disability. Unum recommended she apply for SS Disability they said they would assist her. She ended receiving SS. She told Unum specifically she did not want to receive SS disability until her Unum ran out out in November because she was receiving more income than what SS was going to grant by $400, she wanted to save up as much as she could. Soicial Security ended up back paying mother from the date they originally submitted the claim now Unum says my mother owes them over $11,000 is over pay and is deducting her SS checks until that money is paid back now she gets only $840 a month. Can they do this, how was my mother to know SS was going to back pay her she didn’t even know she was approved until a $8000 check showed up in her account, which she finally figured out it was from SS. Unum even paid her again when her benitfits were to be exprired she contacted them asked what are you doing and they said oh you better send that money back. Do I need to get in touch with a lawyer ?