sharing of information between DWP and Home Office - asking for proof of stay

Hello I am writing on behalf of another.

The person has indefinite leave to remain for over 30 years ago but does not have any documentation to prove apart from a stamp in their (out of date) passport from another country.

They applied for PIP 1 year ago. DWP asked for proof of stay to continue the claim so the passport was sent. They then continued with the claim afterwards. Eventually got to a mandatory reconsideration. At this point after 8-10 weeks they are now asking for proof of stay again and refuse to continue the claim until this is done. This is quite frustrating as they clearly had no issues in the first place and i suspect this might be a favourable decision and so they are pushing this proof.

I am trying to see if the home office can send something to us but on a phone call to them today (by the applicant), they clearly said that if they, the home office,  can access information then DWP can also request this information too.

They said to send in the request by email but without an idea as to timeline. Can't we get DWP to get the information themselves? Is there an email for DWP so I can copy them into correspondence between us and the home office?

Any advice would be much appreciated. 

Thank you

P.S> I am not even sure the home office will respond to the email. the email given was the public.enquiries@homeoffice.gov.uk