The possibilities are almost endless. To narrow things down, your buddy will need to decide if he wants a factory original type finish, an in the wood oil finish, a surface film varnish or urethane finish, etc.
A few years back, Mike Hunter of Hunter Restorations shared with us that he was given inside information that the Winchester Custom Shop used Watco Danish Oil for Model 70's and Model 21's. I don't recall the particular shade. I've used it and it gives an attractive finish on walnut. I stocked up on about a couple dozen cans in assorted shades when a local store was selling it for a few bucks a pint.
