iPhone 8 screen dead after water damage

Hi guys,

I have a friend phone who put his iphone 8 under water (swimming pool) for five mins, he switched the phone off to dry it, then brought to me I noticed there was something wrong with his screen backlight (flickering then stopped but left a spot looked like water) after 3 days the phone was working fine but suddenly screen dead, the phone is working fine and put another screen and is working, I'm just curious If replacing the backlight of the screen would bring it to light up again?

What action should be taking to save screens have been in water? obviously disconntact the battery and drying the board is first things to do but want to know how dry the screen probably.

Thanks.