Having checked the installer's Gas Safe registration prior to the job being started, and with the boiler's benchmark completed you have the installer's full details and you obviously have an invoice with an business address from the guy who installed the boiler and you paid by cheque/bank transfer and you can contact the installer via a landline number or via the person who recommended him. You also now have a copy of the building regs. compliance certificate as the installer has registered the installation.
Or
You didn't check registration, the benchmark is blank, you haven't got an invoice for the installation, you paid cash and you have a pay-as-you-go mobile number and no BR compliance cert
Advice:
Yes you do need to provide an compliance certificate for the owner.
Find a Gas Safe registered engineer and explain what's happened so far - very few registered engineers will be willing to put their name to somebody else's installation.
Might be an idea to post this under plumbing and heating forum rather than GD!