Samsung Galaxy Note 4 — please help.... i have killed the wifes note 4!

I recently bought a used galaxy note 4 for my wife, the phone was bought from a tech guy that worked in a Samsung store.
the phone had been loaded with the note 7 firmware, this was a little glitchy and began crashing the phone.
I assured my wife that I was able to flash the stock software back to the phone, I have done this before on other phones and tablets etc.
the software flashed across with no issues and the phone worked fine..... until my wife restarted the phone, it wouldn't boot up until I removed and replaced the battery, assuming that there was a glitch with the firmware I factory reset and wiped the partition again then re flashed the stock firmware, again the phone worked fine and sailed through the initial set up, once it was up and running I tried to reset the phone to test it.........

now the phone wont turn on, it wont charge, it wont boot in recovery or safe mode.... I have had the battery in and out many times and have held the power button with the battery out to discharge power......
the phone is dead! is there any way back from this or do I accept I need to start looking for a new phone for the wife!!!