If the tires were fairly new when he mounted them, they shouldn't be getting hard or dry-rotted yet, but it depends on the compound. Do they feel hard? Are they sticking to the pavement in braking and turns as much as they used to?

If the tread compound and sidewalls are in good condition, then how's the tread wear? Do the tires have wear indicators? Is the rear very squared off, as ny biker mentioned (as much a product of riding too many straight roads as anything else)? Is he starting to get frequent flats?

Tread life varies hugely depending on the tire and the rider. Unless there are wear indicators, he'll have to visually judge the condition of the tires. Also, typically you'll get twice the mileage from a front tire as from a rear.