Retin-A does not make skin thin, it makes it thinner. Thinner from where it was. Only if someone had thin skin to begin with will it make their skin too thin to wax and frankly they should be using it anyway.
It dehydrates the surface unless they are also using a sodium hyaluronate solution in which case their skin is no more reactive.
Look at them, if they look like they have a new sunburn their skin is not ready to be waxed. This is mostly true of new users.
Really the only time I say no is Accutane (sp) or a non balding head. I will wax anybody's anything as long as they will pay me.