I do feel for the OP's girlfriend. My wife has just been through a very similar situation that started Novemer last year. They tried all the tricks in the book regarding performance and 'procedure'. Unfortunately her company finally made redundancy's and she was one of them while being off sick so unfair dismissal was off the cards.
However, she has now got the weekend to decide whether to accept a tidy sum of money as she took them for lack of 'duty of care' and won. I cannot go into it for legal reasons but if the OP wants to message me, I will be more than happy to advise what route we went down.
If the doctor signs her off sick, she is off sick period and her company needs to address the way they are handling this very seriously or it could be another one that gets stung. I'm assuming she has been there over 1 year?
I would advise she documents all of it and calls Acas as soon as she gets home. They are extremelly good at advising and will appoint a legal representative if she has a good case.
Good luck.