Late in the round, you're called over to a table where the players are in disagreement over a trigger being missed. Anderson controls a
Monastery Mentor and is attacking with a
Brimaz, King of Oreskos and the Cat token it makes. Nishikori, who is tapped out and hellbent, blocks Brimaz with
Siege Rhino. Anderson taps Brimaz and his remaining untapped lands to cast the last card in his hand,
Stoke the Flames, targeting the Rhino. Anderson moves his Stoke and Brimaz to his graveyard, then reaches for his deckbox, pulls out a Monk token and puts it into play, which is where their stories diverge. Anderson claims that he did all this in one motion, that the clock is running low, and he is trying to play quickly to finish the match on time. Nishikori claims combat damage has already happened because there was a pause, then they both placed their creatures in graveyards and he wrote and announced his new life total before Anderson made any indication that the Mentor triggered.
You weren't there when this happened, the tables next to them are empty, and this would be the first time this match that Monastery Mentor triggered. How would you answer this call? What questions would you ask which player? Does something else about the situation (round number, table number, exact time on the clock, sleeve color, cards in hard, lands untapped, spectators present) change how you would handle this?