Stop in for a cup of coffee

Hey guys here is one for a quick opinion. Buddy has a stock 70 Dart with a 340. His alt gauge was not showing a positive. He had it re-built by a very good person her locally. Went through it and all fixed up. Put it on seemed fixed. Drove for a show Saturday and 20 miles down the road again not showing a charge.
Bad alt gauge, battery, fuse, wiring, alternator?? Your thoughts??