Have your friend call Guns Unlimited (402) 339-0771 in Nebraska and ask them about repairing or replacing the rib on the shotgun. Guns Unlimited is basiclly the factory gunsmiths for SKB. The 5 chokes and the sight inserts are what comes with the gun to begin with.
Once you find out what the rib will cost to replace/repair subtract that amount plus at least another $100 from the asking price.
FYI ... a new SKB combo gun like you describe has a MSRP of $3699 and should be able to be purchased new for around $3000. I would suggest that your friend begin his dickering at $2300 before the price reduction for replacing the damaged rib.
As an aside, I have an SKB 605 Combo. The rib on the single barrel became loose (not knocked off). I called Guns Unlimited and was pleased to find out that SKB warrants for the life of the owner. They accepted the barrel, sent it to Japan, sent it back to me - all at no charge other than the cost to get the barrel to them. The barrel came back looking like new a couple of months later. No further problems...Chris told me that all SKBs are warranted for the life of the gun. Pretty neat.
If he told friend to offer just as an O/O without the case than that is way way far to high of a price.
The value of the offering has just dropped by $800 to $1000. The list price of a new O/U from SKB is only $2430 plus the Gra-Coil to make it aprox $2700 as a brand new gun and they are only going to take off a $200? That is a joke ... if he is serious offer them no more than $1600. If your looking for a price comparison there is a place in Texas selling new for $1825 without the Gra-Coil.