Golf rule query - Matchplay

Player A is 1 hole up and on the 18th green.

Player B is also on the 18th green and asks Player A to move his marker one putterhead width.

Player B holes putt.

Player A then holes his putt but without replacing his ball back a putterhead's width.

The error is not realised until both players are back in the bar having a drink.

What is the ruling?

I think loss of hole, providing that the error is realised before teeing off on the next hole (or in this case after the match has concluded) otherwise no penalty.