how far is he willing to drive?
A couple of hours away - but white oak canyon & Old Rag are probably the 2 best trails in the area.
If he goes over to Harper's Ferry, there's the Maryland heights trail.
If he goes to Sugarloaf mountain, there's trails all over the top.
There's the AT trail in Maryland/West VA - Annapolis rocks is pretty.
There's trails all up and down Gambril state park & Catoctin - cunningham falls etc.
For maybe closer in - there's a trail that runs along the Occoquan River in Fairfax county - I can't remember the name of it, but all of that area is park because the Occoquan reservoir is Fairfax's county's water supply. It's fairly long and it might have bull run in the name. It might go through fountainhead park and I think there's a stand of hemlock that it goes through.
Otherwise, there's hiking trails in Mason Neck, but I don't think they're all that long.... Prince William Forest park should have a fair amount of trails.