Phone will not turn on after a screen replacement

Hi, My friend did a screen replacement for another friend of his for the iPhone 6 Plus. This was his first iPhone he has tried repairing, we have tried all possible attempts to get the phone to turn on or show any sign of life but nothing happens.

All the ribbon cables are properly attached and we left the phone to charge overnight as well incase it was a dead battery.

Some spots on the inside of the phone do get hot so does this mean that there is still some life in it?

I've been repairing iPhones for over 3 years now but I haven't had chance to do a 6 Plus, think I'll give it a miss.

Looking for any advice as we don't really know what our last resort is.