Finding a dealer that takes the warranty will help...somewhat. That will reduce your out-of-pocket. And I can guarantee that if you have any medium-to-major-sized repair, you'll pay more than the deductible...or you'll pay something even if there's no deductible.
That's just the way they work. That being said, a dealer or shop that accepts them will already know some of the things they argue about, and may even have a special parts pricing deal with them.
I guess it depends on how much more the person wants for the car because of the warranty. You have to look at how much of the warranty is left, how much it cost in the first place, and then basically pro-rate the price.
Unless the person got totally ripped off, their best warranty should have been around 1200-1500 bucks...that's top dollar.
So if it's a 6yr/75,000 mile warranty, and the car is 3 years old with 40k on it, then you effectively have 3 years and 35k miles left...so I'd pay quite a bit less than 1/2 price for it.
Keep in mind that these warranties' time limits generally start from the in-service date of the vehicle...when the vehicle was first bought new.
So you don't get an additional 5-6 years AFTER the factory warranty expires. They run concurrently, and when the factory warranty expires (usually 3/36) the ESP kicks in.
So a 6/60 warranty is really, in effect, 3/30. Or 3/24, since most warranties expire at 36k.
Also, look at the factory warranties...some cars have powertrain-only warranties that go to a pretty high mileage.
If this person bought the car used and purchased the warranty then, it might not date back to the in-service date...it might just be a flat 2-3 years, xxx-miles deal. In that case, you'd still want to pro-rate it based on how much time/mileage is left.