LL
I don't see any reason why your partner should not write a letter of support.
It needs to be a proper letter, name and address, and they also need to explain who they are and how often they see you.
Then your problems, remember they need to explain the problems that you have completing the PIP activities and that these need to be done reliably and on the majority of days.
Lastly, they need to attest that the information is correct and sign the letter.
I would also recommend that they include details of there credentials.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems