Frueybear
First of all, if there has been a sustained improvement in your mother's conditions then she must notify the DWP
If she was under the age of 65 on 8 April 2013 then notifying the DWP will result in an invitation for her to claim PIP. If she wasn't then her current DLA award will be reassessed.
She should notify the DWP in writing and if possible using a tracked post so that she has proof of delivery.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems