That is like asking what is the price of a blue car. Lots of variables.
First off, who made his Enfield? Longbranch is worth a lot more than an English made rifle. Second, do the numbers on the rifle match, do they match the scope bracket, and is the scope's number on the buttstock?
Of course, condition is also a big consideration. And lastly, what additional items are with it? Is the scope case there? Is the rifle chest there? Those add a lot of value.
A rifle with just a scope and bracket, mismatched, and in used condition with a telescope that may need servicing, is likely around $3500 or maybe less if there is damage. The cream of the crop, a 90L Longbranch in minty shape, with all it's matching goodies, can see up to $15K. Somewhere between those two numbers is the value of your friends Sniper.
A better description, along with some photos, will go a long way to ascertaining value. Besides, all of us Enfield addicts always want to see more photos of these great rifles.
edited to add: Milsurpo beat me to it, but we seem to have said the same thing.