With the caveat that it's worth what a buyer will pay, concensus of opinion put an 1813 in a Gemini stock of similar (but slightly newer vintage) at £1,200. Obviously barrel condition will play a big part, as few people want to buy a rifle only to fork out another £600+ because the barrel was bulged or knackered and has to be replaced immediately. Recent batch testing results will help to prove the barrel is in good nick. The exact sight system and any accessories will make some difference, but £1,000-£1,200 would be a start.
I have to ask, but is your friend sure he has a 1913? The pictures show the rifle has barrel blocks for Unertl-type telescopic sights. These were not fitted as standard to 1913 barrels, but were to early 1813 rifles. Early 1913 barrels were drilled and tapped for the blocks, but as very few shooters now use barrel-mounted scopes these aren't often retro fitted. Bar the foresight mount, and a few years potential use, there's no practical difference between an 1813 and a 1913, so the value won't be very much different if it''s an 1813.