Southgate was a very prolific contemporary builder from Tennessee who was active from the late 1940s till late 1970s. He succumb to the effects of a stroke and passed away in 1979. His styling was in no way up to the standards of todays makers but despite of this they were a good gun and highly prize in the day. When I started muzzle loading back in the early seventies at local matches a good offhand shooter toting a Southgate would likely finish in the winner's circle. South built over a thousand muzzle loaders and would have likely built quite a few more if not for the stroke. He put serial numbers on them all, usually on a barrel flat near the breech. Dixie Gun Works, in the back of their catalog, have the serial number range listed along with the year. If you don't have access to the catalog and you find the number let me know and I'll be glad to pass it on to you.
As far as a fair price, of course, it's up to the market. There is a small niche of collectors interested in them but I don't know any of them. Your friends rifle, while appearing to be in great shape, is still a fairly plain example. I've seen in recent times much fancier Southgates bring around $1500. Just MHO the one shown would bring much less. Hope this helps a little and good luck to your sale.