It is very important that you give the insulin after dinner!
If you give it before, and the cat, for whatever reason doesn't eat, it can cause hypoglycemia, meaning the blood sugar becomes too low, also called insulin shock.
It can be deadly.
If the kitty doesn't eat, do not give the insulin and call the owner or vet about what to do. Sometimes a reduced dose of insulin can be given if the cat is not eating.
But I would not do it without the advice of a veterinarian.
An hour early or late shouldn't matter, as long the kitty had dinner.
I'm surprised that the owner didn't leave you instructions, since this can be a life-or-death situation!