He could do worse than take a trip to Guildford, Surrey, my nearest town, and find Ben's Collectors Records. This is not a specialist jazz store, but Ben is a real enthusiast, and a very nice guy, and always has an enormous used jazz selection, on CD and LP, and at very keen prices. He buys and sells, frequently buying complete jazz collections on LP.
To find it, walk up the old cobbled High Street to the big 17th Century clock, turn right under the arch into Tunsgate, walk up and Ben's is the last store on the right.
Search and you'll find his web site.