Looks re-barrelled to me and original proofs for the new barrel.

Your friend should follow Daryls advice and contact Hollands first to establish the history and mate(s) of the gun. This narrows the search down first and lets you know what gun number(s) you are actually looking for.

Matchedpairs.com had been the main source for reuniting pairs in the past, but I believe the advances of archive auction house listing on the net and constructive google searches have made the service that Joe offers almost redundant.

I have had some success in reuniting a few Dickson guns this way (all big auction houses have an archive online) and one site that is really good for trawling the archives is artfact.com where you can even specify a serial number.