The solicitor is being an arsehole, I had exactly the same situation mate a few years ago. My mate and another mate of his had fitted their own combi and rads in a holiday home they bought in the lake district, about 5 years later the mate rang me and asked if I could sign the boiler off because when he'd filled the forms in for the exchange of contract he had put down that the combi had been fitted in the last five years. The solicitor had noticed this and wanted the building regs cert. Anyway I refused to register BUT I wrote him a very vague unspecific letter with no details saying that I had worked at the property during it's renovation and had added parts on to an already existing system. I never said what parts, and never heard another thing from the solicitor. Worse case for your mate like others have said is that he will need to take out indemnity to protect the new owner, more and more solicitors these days are picking over details.
When I sold my own house a few years ago, 'their' solicitor also wanted me to buy an indemnity policy for my double glazed windows because building regs hadn't been notified when they had been put in, they were already in when I bought the house about ten years before and luckily for me I had kept the previous owners list of fixtures and fittings and on there he had wrote DG fitted 1990. Otherwise I would've been buying indemnity, it's a ferking racket !!!!!
