And how much of the ATPL requires you to know how to integrate, partial differentiation or what the heck are complex numbers. How much of the ATPL requires you to understand thermodynamics or metal removal rates of a tungsten carbide drill!
The ATPL theory requires that you can add, subtract, multiply, divide, understand the basics of algebra and trigonometry. Nothing that is beyond any O level Maths students grasp.
If the guy wants to study pygmy and sumarian architecture for his A levels, then let him do what he enjoys and don't fill his head with scary stories of how hard the ATPL's are. It the quantity of knowledge that is daunting, not the quality.