Sounds like a fuel delivery problem to me as well.
Could his lift pump have died? On my old 94 there was a lift pump on the frame rail under the drivers side.
I upgrade to the Tekonsha Prodigy P3 last year...and will NEVER go back.
I had borrowed my neighbors truck (which had a prodigy) to pull my trailer...wow, what a difference.
I bought one the following week and love it.
It has to be mounted in a somewhat horizontal way, or else the G-force...
Good work.
While I dont have a dually, I redid mine a year or two ago, and found similar results with cracked pads...that I attributed to "performance" driving habits...I have since mellowed down on stomping on the brakes.
I havent done this mod yet, I had planned to put an aftermarket intake on, but never got around to it.
Interesting if it really makes that much of a difference.