The ECU is likely reparable, in any ignition module the power transistors usually fail first. ( The " ignition module " is integrated into the ECU )

There are 2 coil boot failure modes.

One is where the resistor loses conductivity.. It may arc across for a while but eventually it will open. When it opens it will take out 2 cylinders and eventually the coil.

The other is where the boot punctures and the electrical energy never makes its way to the spark plug. In this case it will take out the cylinder attached to the boot but not the one what uses a plug wire. ( 2 and 5 for example )

All of this applies to the plug wire as well.

He got a code because apparently the ECU monitors ignition coil current or the inductive kick when a plug fires.

Another wrinkle. On my 97 C280 and SL320, if there is a specific cylinder miss fire for say 15 seconds, it will shut off the injector to that cylinder to prevent cat converter damage. This can cloud diagnosis when dealing with an intermittent ignition miss fire.