Computer stuck at 640 resolution

I recently built a computer for a friend and I ran into a really odd issue...

I have everything up to date, including drivers, and it works absolutely fine on my monitor. He came over today to pick it up, and I have a secondary area for computers I built for testing so I figured I'd boot it up and give him a test run so he could see what it was like.

When it starts, the monitor doesn't turn on until windows Is already booted in (No windows login screen, no motherboard screen) and when it does start, it's stuck at a very low resolution - 640 x something, I honestly don't remember the x because I was pretty panicked. I was unable to manually change the resolution, to even select it was entirely greyed out.

I then brought the computer back into my main room and plugged it into my monitor, and it worked just fine. I thought ok, maybe this was just some odd screwup so I brought it back into the other room to test with his monitor, and it went right back to the 640 issue.

My monitor is a 1080, and his is a 1440x900 the only thing I could begin to think is that I set it up on my monitor, and maybe setting it up on a 1080 monitor is somehow meshing with plugging it into a 900 monitor, so I brought it back into my main room, manually set the resolution to 1440x900, then brought it back into the testing room. It still persists.

I can verify that both monitors are working (He was using it before coming to get his new computer) and as I said, all of the updates are recent including drivers and it's 100% functional on my monitor. Any idea what this could be?

What's the parts list look like. Having GPU/CPU/RAM configuration information might help diagnose the problem. It could also be an issue where you need to install his monitor's drivers on the system for it to recognize the screen resolution properly. Some old LCD screens didn't have ESSID built into the driver board and required an OS level driver for the system to recognize the full resolution of the monitor.

Probably best just to reinstall everything while hooked up to the new display

Or try running a live GNU/Linux USB on it to see what comes up then, Gparted live would work as the file is small, then just use rufus to make a bootable USB

If you are using different ports on your GPU for the monitors then that may be the problem. You could try hooking up your monitor and the spare monitor at the same time and see what happens.
If that doesn't get you anywhere I would recommend following Streetguru's advice.

He got a new monitor and it works now. I think when he was bringing it over to my house it may have gotten knocked around in his car or something. I don't know how that would result in it only being able to display low res, but who knows. At least he's able to use it now.