There used to be a good secondhand bookstore near Jazz Record Mart, just off Michigan Av. I don't know if either shop is still in business. Also don't know of any good military bookstores, although there is probably a good shop at Cantigny Park (I haven't been there since I was a kid, even though I went to C.O.D. and drove past every morning for two years). I know the whereabouts of more record shops than bookstores. If your friend's looking for specific books or just wants to browse for US market stuff that's harder to get in the UK, he'd be better off going to the Michigan Avenue Borders, which is enormous.
If needed I can also give the names of several good restaurants in the city and near environs. My other specialty.
-Wm.