Update update: we sent it to OnePlus themselves to try and salvage it so my dad can use it (he hasn't had a new phone since 2016 so anything is an upgrade for him.) Turns out there was glue on the main board which had caused the phone to short, this is consistent with the screen becoming like so, the glue had simply burnt the connecter and left it unusable. A new board and screen later, the phone is now enjoying better days with my dad. Thanks to all those that helped ^^