He wants to tinker. Let him tinker

If he was cutting chunks out of his rotors to save weight, I'd give him a hard time. But he's doing something that manufacturers sometimes do anyway. Like adding sound insulation or dynamatting the doors. Surely, any amount of added insulation would cut back on noise. But the job has to be executed well. Cleaning everything will take much longer than spraying it on. Personally, I agree though. The car is built how it is. But i don't see any harm here (Unless I'm TOTALLY misunderstanding what he's doing)
I think he just wants to spray the sheet metal in the rear wheel wells (not any plastics)
I'd like to point out that the rear portion of the wheel wells is plastic from a line basically just forward of the exact top of the tire down to the skirt at the bottom of the bumper. Water that comes off of the tire should be gone by the time the tire reaches the top, and any water that still sprays off will be pushed by air towards the back of the car, not drawn toward the front.
The rear wheel wells do have metal in the forward half that is also a structural barrier for the fuel tank, support for the doors, and whatever structural purpose one could dream up, but the metal is already coated with rust proofing, and is galvanized. Dealers sell sound proofing and underbody sealant for purely profiteering reasons. The automaker (any of them, and surely BMW/Mercedes, and the like, has to be an industry leader here) will protect the very same areas that the dealer wants you to buy protection for. The dealer put the car on the rack and takes the tires off, and tapes around the painted surfaces, and somebody making $20 per hour takes about 10 minutes (or less) to spray about $15 of material (retail pricing) onto the bottom of the car, and they collect $500 for the service. Pure profit. Don't forget the Scotchgard while you are blowing money...
Every consumer report that tells you how to avoid being fleeced on a new car purchase will tell you to NOT buy the underbody treatment. This is not all they say, but they always say it.
Having said that, if there was ever a good time to buy underbody treatment, it would be on the very first day when the car is still clean.