There is as many ways to paint a car as there is painters.
Stripped and on stilts, Soooo.... complete underside, trunk area, complete interior, engine compartment, and exterior and whatever is loose. That's alot of surface to maintain a wet edge and get around on one shot without having dry spray lay back on the interior floors or trunk floor or blow back out on the exterior. You'll have quite a bit of time just cleaning the shell after prep.
Most painters would break this down to trim out then paint, do the interior then back tape the holes, strategically done so there is no lines and it would provide a cleaner job.Requiring not only masking but several trips through the booth depending how it's done. Probably triple the surface area of just an exterior along with the materials and you still have to rub your fingers over every square inch.
Not to argue the 15k figure, for whats involved here, but there more materials and labor involved then what first meets the eye. it's sounds like he wants a top notch job with quality materials. Hopefully that what he gets