Canned food is cooked in the can. I don't think it'll hurt it to get cooked again. Although it would depend how long. Was it just the 15-minute preheating time? Haha, I store pans in my oven and I can't count how many times they've been preheated. If I kept cat food in there I'm sure the same thing would happen.
I guess I'd still use it, unless the cans blew up, but I'd try to use it very quickly.
I don't think it would hurt if it was just a short bake. I don't know if the additional cooking would cause the nutrient breakdown. If the consistency of the food has changed or it looks at all different I would toss it.
I'm not sure how long he cooked them for I'll ask him as soon as he comes back. I opened one of the cans and it looks pretty normal just that it's slightly drier and its scent isn't as strong.