I don't think that will fix your problem, but most power supplies have an adjustment. He'll either have the standard type power supply that you'd find in most games that will have a knob on the top above the screw terminals, or he'll have a big silver box that usually has a knob sticking out one side that adjusts the 5v.
If he turns it up too high he can damage the game board and the guns, so he should measure it first.
If the game is booting, though and running, the 5v is probably close enough that the guns should work right. He'd probably do better by adjusting the brightness on the monitor, if the monitor doesn't have good contrast the gun won't 'see' the flash good enough to make it work right.