I'd suggest that she talk to a lawyer. I'm not sure that they can force her to go to substance abuse counseling, after a test is clean. I don't believe that just saying you've smoked in the past is enough to mandate counseling.

Also, it is clear to me that your mom is and will continue to be harassed by management. If a lawyer agrees that she has a case, they might work for a percentage of the damages.

Whatever you do, make sure you continue to help her as much as possible. She is under alot of stress, and has lost much of her support system. Assure her that she'll get through this. Life is hard sometimes, but there are great rewards once you overcome the hurdles.