Double check what a new one cost, I think the assain is a RTS bow (Ready to shoot) so it comes with sight, rest, etc. installed. I want to say their around $500 new. If he has a bunch of other gear with it (broadheads, arrows, release, etc.) its a good deal! Check ebay too, I got my Bowtech Invasion for $800 rigged out with a ton of accessories. Brank new it was over $900 for just the bow!
Don't listen to the matthews guys, they've drank the koolaide and theirs no going back!