Black Screen After Third Party Replacement, Phone Powers On

So, a friend gave me their iPhone 6 that had a third party screen repair. The screen worked on another device at the third party repair location. The repair shop tried a different screen and it did not work on the phone. The repair shop guy said that sometimes the motherboards go bad in the phones, but the phone still powers on and makes noises. Is the phone screwed? My friend would like their stuff off the phone but I cannot get the screen to work to retrieve their info.