It is not permissible to use borrowed sails in the way he describes as that would be contrary to the rules.
The boat must be measured with its own sails when measuring for the purposes of getting a certificate.
Whilst it is useful to check a boat with borrowed sails ( I keep some for that purpose) it only establishes what can be fitted to the boat.
Once the proper sails are made the boat must then be measured with the proper sails.
Tedious but true and has certainly always been my practice.