I just noticed it says digital display even without the all-in-one connected to computer
This is a weird idea. I think that in order to get it to work you will have to perform open heart surgery on that other PC, remove the display from it, then scab it back on to a power supply/input source. I cannot see any way that you are going to get this to work. Build in displays are set to only read the one input (the computer itself) until booted, where they will (perhaps) recognize others. You are most likely better off selling the all-in-one, and buying a new 22in LG monitor to match.