Welcome:
In my opinion, the hand is dead. The player is not present and therefore cannot win the hand. The fact that he is "all-in" is not material.
I would not, I repeat not, consider this as a typical "all-in" situation where the cards have to be turned face up as soon as all betting action is complete. There is no point in turning the cards face up as his hand should have been mucked as soon as the last 2nd hole card is dealt to the button player.
This players chips go in the pot, his cards go in the muck and the hand plays out.
Chet