I don't think it's possible that Josy went to that dorm room to see the MC. When Josy calls him, they have the following conversation:
MC: "I don't know what to say..."
Josy: "You don't have to say anything... ...except... ...tell me your dorm number."
MC: "Why do you ask?"
Josy: "Well, I was thinking... Maybe...until I get my own dorm...I could stay with you?"
At that point there is a choice for the MC to say that he is staying with a girl, or just that he is living with someone and there is no room. Either way Josy responds understandingly and says,
Josy: "I can always stay with a friend or something."
Implying that the 'friend' is Maya. While Josy does ask directions from the MC, he never provides them.
It also seems to me that Josy would have already known where Maya's dorm room was. She may have already contacted Maya to say she was coming. Just not when. I don't really know about that. But she did no where to go, just not from the MC.
So Josy's stunned look was not because Maya opened that door, but because the MC was there, and Josy suddenly realizing that Maya was the person the MC was staying/living with.