Make sure his insurance company didnt pay out, ie., salvage title. If you can "steal it" (buy it cheap enough that you wouldn't get hurt if something major was wrong) then you'll prolly be ok. I would be concerned that he rode it home after the crash and there may be oil starvation damage, be hard to lose all the oil with just a side cover crack.
You're probably the only one who has called on his bike,, I can't imagine a 10 year old wrecked FZ1 would be in high demand. Negotiate accordingly. Good luck!