S6 repair, plausable or con?

Please can anybody with experience of galaxy s6 repair help.

A couple of weeks ago I turned of my s6 one night, next morning I tried to turn it on but it wouldn't, it was completely unresponsive, black screen no lights, the phone has never been dropped or damaged in anyway or been anywhere near water.

When it was switched off the battery was almost fully charged, so I knew it wasn't a dead battery but I tried the charger anyway. There was no charging light and the back of the phone near the rear camera got quite hot within a couple of minutes. I tried a soft reset but it didn't work.

So then I took it to a local phone shop for repair. The guy in the shop said that he'll send the phone of the workshop for a diagnostic. Anyway he called up and said the motherboard is faulty and needed to replaced. Now he phoned up again and the guy who is fixing the phone is now saying the screen also needs to be replaced, apparently the backlight is not working. Now I know amoled screens do not have any backlight, so I asked him what exactly is wrong with the screen as it's a amoled screen it doesn't have a back light, he said he doesn't know just the engineer said the backlight doesn't work. He said the repair would cost £149.

My question can a faulty motherboard damage the screen? is he what he is saying plausible or is he trying a con?

Thanks in advance for any reply's.