Well, I don't have PZero Nero M+S yet, but will be buying them in the spring strictly based on Tire Rack's test data. They have a 400 treadwear rating compared to a 360 for the stock Dunlops. The treadwear rating doesn't mean jack when comparing tires from different manufacturers, but I would think the Pirellis will last longer, especially if he's diligent about rotating them.
I base that on the knowledge that OEM tires are often softer than their aftermarket replacements -- even the exact same tire. I don't drive very aggressively and my stock Dunlops are down to 3/32's of tread left at 12,000 miles. You should get 20-30k out of them! And that tread depth is across the entire tire, so I haven't worn the shoulders down faster.
Pirelli makes good tires, and the Nero M+S is highly rated. Go with the 205/50 size and he'll be fine.