Adam,
I've visited about 5 in the DC metro area which includes Montgomery county MD, Frederick, MD, Prince Georges County, Md and Northern VA. I like Golden Eagle (bought 5 SAE rolls in 2008 for $395, now the're selling for over $800/roll) in Laurel, MD. However most of my buys have been at some of the good shows at Dulles Coin Show, one at a local school right off of the 495 beltway (can't remember the name) and from a show up in the Mt Airy, MD area. Thats the one put together by Mike D who is a collector and schedules shows.
Tell your friend to do a google search for coin shows in VA, then MD. Never seen one in DC. Upcoming shows will pop up.
Whats he collect? He can pm me. Good luck.