2nd monitor not turning on

Hey, my friend is having an issue that's confusing the hell out of me.

As the title says, his 2nd monitor won't turn on. Luckily, he has another pc system that we can swap parts out with no problem.

This led us swapping out his monitor to see if it's the monitor working our way all the way down to the motherboard.

The questionable monitor works fine on the other computers, just not his.

We swapped his GPU around to another system, monitor works fine on that system. Put it back into his, it fails. Even changed out the cables.

He just replaced the power supply and the motherboard, and the processor is fairly new as well. I've not heard of a processor ever being the cause of why one monitor will turn on but the other won't, and I'm out of ideas.

Any help would be appreciated.