The valve covers are notorious for leaking. Did he use the cork felpro-gaskets? The Cork stuff is known to keep leaking even with new gaskets. Use the Ford replacements, or the felpro Perma-Dry gaskets.
I just had the same probem today. I had my valve covers off last week, and ive noticed some oil (about a tablespoon) everywhere i park. I narrowed it down to the valve covers.
10mm socket and 3/8, problem solved. Although, be VERY careful on the top row of bolts. Ford specs those to be like 3ftlbs for a reason, the lower intake is ALUMINUM! Ive had to heli-coil two bolt holes for the valve covers on that stupid intake already. The lower side (heads) will give you a little more room for error, as some heads came cast iron. If the leaking is comming from the lower-side of the valve cover, i would just tighten them down until it feels good. that should solve it.
To single out the rear main, look behind the engine. If oil trails down the tranny and has lines spotted on the Exhaust Y-pipe, you can nix the rear main. If the oil is comming out of the small hole on the bottom of the tranny, where the tranny meets the oil pan, then good chance its comming from that rear main.