Can the original owner get his account back?

[Barbar]

So i'm just wondering, I have a level 6 unverified account that has a last play on it from 2014. My question is, if I was to add my own email to the account and change the pass, can the original owner ever get the account back? Like say if they contact riot? I'm just trying to think if he contacts riot how can he prove the account is his?