My understanding is that the person who is pressing the buttons gets the money. I asked Brian, who helped with the last question, about this. Here is what he wrote, which I agree with.
In the scenario described, the person who put in the money and pressed the buttons would receive the jackpot.
What I find interesting about this question is the paradox that in all likelihood, the jackpot never would have occurred without this chance encounter.
As you know, the random number generator in the slot machine is continuously working even when the machine is not in play. So even though one patron feels cheated, their run-in ultimately led to pressing the spin button at that exact millisecond when the RNG was on the winning combination. So, if one patron had acquiesced, there is never a jackpot to fight over.