I think the last word he says to her before she goes to the washroom at his place is 'suprise', and they stressed the word to sound just like he said it when he was in the suit a couple times
That's what told me it was him, and probably triggered her too
Even though he said that, I still had my reservations about it actually being him. Weird, I know, but during the entire film, we never actually saw him do anything bad to her. I mean, I guess when he ran to the car at the very beginning that was supposed to show how "villainous" he was, but for the most part, she told us everything he did with nothing to show for it. That's really my main complaint...never seeing dude in the act.