Any chance he paid by credit card? Might be able to cancel the payment via card company?
Agree that posting the certificate up here will let us spot glaring obvious mistakes, but some items may need an adjoining photograph of the installation as well.
As mentioned, blank off all personal details… we can’t have this companies name on a public forum just to criticise them.
Is the company part of a competent persons scheme? Ie, NICEIC, NAPIT etc? A trade body that can be complained to? Not that they’ll do much unless said company is using the c.p.s branding without membership. (Also likely)
A photograph of the consumer unit is always good. There are recent changes to the regulations as to what should now be included.
I’m sure you know what you’re doing, but as a final comment, please be careful poking about.. there could be more live open ends ready to give you a bite..