Hard to come up with a reasonable price with the only information on the gun's condition being 7-8 years old. So I have to base my assumptions on there being no modifications and he still has the case the gun originally came in.
$250 is a realistic selling price assuming the case and two magazines. I'd strongly suggest, however, starting with the dealer he's buying his new weapon from and seeing how close to that target the shop is willing to give in credit towards the new gun, as the amount of time and effort saved would justify it.
If there's been significant upgrades, and those upgrades have been professionally done, the shop is most likely the best place to recover those investments, as they'd be best able to get the premium price vs some other sales method.
In any case, if you look at it as $50 a year use out of the gun for 8 years, he's already gotten $400 back off of it, which exceeds the value of it new; any more money back is just creme on top.
As for the PPS, my advice is only buy from someone who has had a long experience in selling and servicing Walthers; too often I've come across great deals from sellers wanting to dump a less than prime gun that only really needs a little TLC to make into a great weapon.
The S&W is probably a better choice if he doesn't have a good gunsmith to work with.
In either case, he should see if a local range has any on hand to try, or check with friends at the shooting range. Both guns are dramatically different compared to the Taurus if he's put significant amounts of lead through the gun.
Good advice. Thank you.