I sold an immaculate Mercury S on here a few years back for £200 but that also included a service kit with it. Ordinarily I'd say quite a bit less than that for a standard Mercury, but if your friend's rifle is in excellent nick and with original box that'll lift the price.
I don't want to break any forum rules by asking if you've photos to show, but I guess they'd help the experts give a more accurate valuation.