When I registered it asked me for more information - it said it was unable to find enough to support my claim of living here. The online system gave several examples of documents that would suffice, and I was able to choose which ones I wanted to use.
Apparently, when I registered (during the test phase) the system was still not able to access HMRC records to check for tax status/activity or whatever agency it is that handles the govmt. pension contributions here. So I had to upload scans of my council tax bills, which I did. Took me about 5 minutes to scan them on my scanner and then do the upload. So now they have proof for 2017, 2018, and 2019 for me. I would imagine that when I file for "settled" (and not pre-settled) I'll have to upload them again.
If he's been working here for years, perhaps they still can't access those databases? They do seem to have some problems with the implementation of this, now that it's gone "live".