DLA - PIP transfer claimant (Northern Ireland)

I have a client who is severely sight impaired and did not receive any audio correspondence when asked to migrate to PIP. He therefore did not return the PIP2, as he didn’t know he’d received it. I’ve seen some information somewhere which was released by DWP after a freedom of information request about the procedure to be followed. Has anyone got a copy of the Northern Ireland guidance I can use, or any other information, which might be useful in my backdating request? Thanks.