I am concerned he hit the battery side of the fuse. That could have pulled a lot of power from the battery, and strange stuff can happen with a short.
In a 65 A body, the lighter socket center contact came un riveted and shorted out when I unplugged a phone charger. Not only did it take out the fuse for the lighter, it took out 5 other fuses, and the aftermarket tach still has periodic tremors. I added a fuse in the line to the lighter socket, but am still wondering how it pulled that much power from the radio etc.
There may be some investigative procedures needed on this Mustang.