iPhone SE showing no signs of life. Think it could be software related

I have a problem with an iPhone SE which I really don't know what to do. The owner of the phone told me they were in a busy area with Bluetooth on and the phone keep asking about signing into random iTunes accounts they'd never heard off and then it turned off and wouldn’t come back on.

I have tried to put the phone in dfu mode. I have tried connecting a new battery to it but still no signs of life. Nothing happens when connecting it to a pc or charging socket.

Has anybody had this problem before or knows how to flash an ios onto the a dead phone in case it had it's software corrupted?

Thanks