If you look at the form, it does have a space for the referee to explain why he can't give passport or naturalisation certificate details.
If he couldn't provide either set of details, I'd suggest you found another referee but, given that he has a passport, surely the commonsense answer must be to use the space provided, and write "I have lost my naturalisation certificate" in the box that is there for the purpose?

paul

British born citizens will not have a naturalisation/registration certificate number. If your referee is British born, then only the passport number is required, the rest can be put as "N/A".

*My current/expired British citizen passport number is <<passport number>>

*My naturalisation/registration certificate number is <<N/A>>

(British citizens by naturalisation or registration only)

*I am unable to give details of my passport/naturalisation/registration certificate because

<<N/A>>

P Pron & Mrs G would be able to correct me if I am wrong.

Looks fine to me!

paul

What should I do when my referee has only his passport but has lost his certificate of naturalisation?