Hi Tyra,
It is probably too late to post the report and get the Assessment Provider to call off the appointment as a result.
I suggest your mum* phones the Assessment Provider on Monday morning and explains the contents of the letter. There is a chance (although, IMHO, a slight one) that this might do the trick.
If the appointment does go ahead, you/your mum should give a copy of the GP letter to the assessor as soon as s/he arrives.
* Or someone on her behalf although your mum would have to answer security questions first and give permission for someone else to speak on her behalf.
Nothing on this board constitutes legal advice - always consult a professional about specific problems