OK, here's my GUESS on what happened.
The car had a fuel pump issue previously - as well as a plugged fuel filter. Both were replaced just before he started working on THIS problem.
My SUSPISCION is the engine was running a bit lean due to poor fuel delivery, so the computer compensated by increasing the duty cycle of the injectors - and the guy kept driving it.
High duty cycle means higher average injector current, which means more heat. The injectors overheated and the insulation in the coils broke down, causing the injector coils to short when warm - lowering their resistance.
So on a warm start, in particular, but possibly even on a cold start, the injector resistance was below spec, and the currrent over spec - causing the injector driver in the computer to shut down on overload protect. Disconnecting one injector reduced the current so the driver came back "on-line" and fired the injectors. At speed,(higher frequency) the inductance of the injectors was higher than the DC resistance, so the current did not get into the overload range, and the driver kept pulsing the injectors.
This is my suspicion from what I HAVE been told. I have not been told for sure what he ended up doing to fix it, but since I suggested this might be the problem, and he has not gotten back to tell me I was wrong (you know how younger brothers are????), the chances are pretty good I hit the nail on the head.