I have the opportunity to buy an 1851 Navy .44 with the brass. In a hard case with everything needed to shoot it. Seller hasn't giving a price, kinda asked what I would pay(family friend). Powder, flask, wads, ball, caps, capper tool, etc. In great shape, also don't know what to offer.
You have to be careful with friends and family. I would not make an offer. It is his property, therefore have him provide a price. You can negotiate from there if desired.
A long time friend of mine one time had an old 5-screw S & W he wanted to sell. Asked what I would give him for it. I did a www search and forwarded the info for him. It was apparently far less that what he was expecting. The guy was insulted, and wouldn't talk to me for at least a year (which showed me what kind of friend he was).
From the information above you should be able to determine what that gun is worth. Have him provide the price. Go from there...