ECU: What should the arbiter do? Forced moves in arbiter practice

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By IA Prodromos Gerontopoulos (GRE) 

In a standard game (90 minutes for the first 40 moves followed by 30 minutes for the rest of the game with an addition of 30 seconds per move starting from move one), the following position (diagram 1) occurred in the presence of the arbiter after white's 46th move.

Position after White's 46th move 

The player with the black pieces played 46...Rc1+ but  his flag fell before he had the chance to stop his time. The player with the white pieces immediately called the arbiter and claimed a win. What should the arbiter's decision be?

 You can find the solution on chessbase.

Forced moves in arbiter practice - Gerontopoulos Pr.