iPhone 6s died after a night charge

Yesterday a cumstomer brought a dead iPhone 6s after a night charge, the phone is only 4 months old and apple refuse to accept the phone for a repair or swap.

I advised her to contact Apple customer service, since the phone is only 4 months old.

She told me that she’s no longer wanted to return the phone to the Apple store, because how they treated there customers.

Before I explain the problem, the phone has never been opened before and not touch with water.

I opend the phone and I start with test the battery with the multimeter. I find out that there is no voltage anymore on the original battery.I replace the battery for a new OEM Battery and the phone turns on again ☺, but he didn’t pass the apple logo. The phone stays on and only shows the apple logo.I connect the phone to Itunes and try restore the phone with recovery mode and dfu mode, I get the message error 6. I open the phone again and disconnect the rear camera and proximity sensor and try to restore again with no lock. I try different original apple usb cable and difference usb ports but with no luck.

I think there’s a broken component on the motherboard that need to be replace, but I don’t know where I need to start.

Can someone help me with this problem?

By the Way sorry for my bad English.

Best regards,

Marvin