If your friend crossed two spark plug wires, there is no way he could idle fine and have no problems up to 40 mph.

BTW, you were given good advice to just change the spark plug wires one at a time, if you haven't changed them yet. I would add one note: Most sets of spark wires are pre-cut to the proper length. When replacing the wires remove the longest one first, and replace it with the longest one in the new set. Then work down to the next longest, and so on.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.