I'd just explain it exactly as you did, that she's been in your sight to prevent it, and she didn't cry or say anything that would make you think she was being bitten. In my experience kids lose their freaking minds when they're bit, regardless of how much it hurt. I think you'd have noticed. Maybe ask if there's a possibility that she could have been bitten elsewhere or fallen on a toy.

Maybe also ask mom if they have any other concerns. This could be dads [crappy] way of dealing with something else that's bothering them. I know you mentioned they weren't happy with the new changes, if they're going to try to be looking for reasons to pull her, I'd try to get ahead of it.