The documentation listed as acceptable on uk.gov is:
• Your passport; and • Your full birth certificate (one which includes the name of your parent(s), (which should be requested from the relevant authorities in your country of origin); and • Your mother’s full birth certificate; and either • Her certificate of naturalisation or registration as a citizen of the United Kingdom and Colonies (or, before 1 January 1949, as a British subject); or • Papers showing her legal adoption; or • Her expired citizen of the United Kingdom and Colonies passport.
The problem is, his mother died in 1979, when he was a teenager. Due to circumstances beyond his control, her birth certificate and passport are lost. I know we can order a copy of the birth certificate (hopefully as easy as paying the cost of it and shipping, or are there hoops to jump through?), and as she was born in the UK, I can't see her having any other naturalization or registration documents.
Her passport was thrown away by her husband's next wife, and so is unrecoverable. Can one order a copy of her passport, or do I need it if her birth certificate shows she was born in England? I don't see how they can expect people to have their parents' passports. What happens if their parents are dead? Do we really need her passport? Do we have a chance of making a successful application?