iPhone dead but apple techs tested it and say it's fine

About six months ago I replaced the broken screen of my friend's iPhone 6 with a refurbished one. It worked fine after that but now it suddenly turned off and won't turn on.

I sent him to an Apple store. According to my friend they hooked a device to the charger port, he was asked for his pin and everything seemed to work. But the screen will not turn on because, according to them, it's a fake. Also because of this they refuse to fix it.

I've replaced a bunch of iPhone screens for family and friends and never had this happen before.

I tested an original working screen (a working screen with a broken digitizer) but it still won't turn on. If it was just the screen, the phone would at least vibrate.

Could it be the battery? The charging port seems to work (otherwise the apple tech wouldn't have been able to test the phone).

It seems unlikely but I'm no expert so is it possible that a refurbished or maybe fake screen could damage the motherboard (graphic chip or something)?