Once a paint has cured, you can't add more. The new coats won't adhere correctly and you'll have one heck of an ugly mess.
Now, a body or paint shop could do it, but by the time you pay them to prep it correctly, it's going to be about the same price as a full repaint anyway. That's the big cost in paint work: all the prep that must be done before you spray.
doing the sanding and "prep" required for simply repainting isn't very costly if the car has nothing that needs to be fixed.
removing headlights, trim, taillights, etc is going to take as much time as doing the sanding work on the body. taping off and painting.
the fact that he wouldn't need bodywork would save a ton of money.
shop around, find a place that can shoot a new base and clear, and you will find you could have it done for anywhere between 1200-2000. the paint will only cost around 300, but the time it takes to prep, spray, and reassemble is what is costly.
if you are confident with your mechanic skills, then take the steps to remove everything you want out of the way (headlights, taillights, etc etc). tow the car to the shop since you wont be driving it, have them finish their side of sanding, taping, spraying etc. and then after its cured enough to be outside (2 hours about), have it towed home and start reassembling.
you can save a lot of money by doing a lot of the prep yourself.