Kathleen suspects the truth about Joe being her online buddy since shortly after their meeting in the coffee shop.
When he finally reveals himself at the end, she looks unsurprised and says "I wanted it to be you. I wanted it to be you so badly"—implying she'd suspected the truth for a while. But for how long? I think there's a case to be made that her suspicions began shortly following their meeting at the coffee shop. She doesn't seem to realize it at the time the meeting happens—but when reflecting on the experience later, wouldn't she think it strange that Joe just happened to be there, unaccompanied by anyone else, at the exact point when her online friend was supposed to show up but didn't, let alone why he took so much interest in her? At the very least it must have crossed her mind.
