What to do when owner will not provide proof of vaccinations for a boarder?

I have a proof of current vaccination policy for my boarders. Generally not a problem, but I have dog coming Wed and the owner has still not provided. He messaged me on Monday saying he can't find it but assures me the dog is up to date. I told him all he had to do was get his vet to email him a copy and send me a photo of it. He said Ok, but I still haven't received it. I plan to message him tomorrow telling him if I don't get the copy by Monday, I will have to cancel. I hate to threaten, so does Rover have advice on a better way to handle this? Thanks.