To be honest it really depends on how well it’s been looked after / wear and tear.
From what you’ve said sounds like it’s got little wear so I usually go by if it’s good condition 2nd hand then a little less than half the new price, if it’s very well looked after then half new price and if it’s totally mint shiny new condition then enough less than he could purchase equal quality kit to make it worth not having new.
Hope that makes sense – also there’s all the factors of your situation and his as far as money goes – at the end of the day between friends it’s a price that both of you are happy with.
I did a little google search and found a more used sounding one going on Bikemagic for £750 ono
All the best 🙂