Need help determining if the screen or motherboard is the problem

Just so I am clear that I am I am interpreting the issue correctly. Your friend had a phone with a broken screen, bought a phone from eBay to use the screen from? You swapped the screens, doesn't get an image with the new screen? Is that right?

Regardless, I think checking to see if the original display still works would be a good next step. Unless it was also having the same original symptoms. If the original display still works. It sounds more likely that it's an issue with the replacement screen. If not, or the new display didn't resolve the initial issue, I would inspect the connector on the phone and any components around it. See if anything looks amiss. If you can't tell, feel free to upload some photos to your original question. If the new display was harvested from a "for parts" phone, anything could have been wrong with it.