I'm not sure she even realized at the time that there was reason for alarm: there's no suggestion that she saw blood, or a weapon.
If you're living in a house with a number of other students, who entertain friends (and take food deliveries) at all hours, it might not be that unusual to hear someone crying in the night, or to open your door at 4am and see someone you don't recognize. She may simply have peeked out the door -- half-asleep and annoyed at the noises that kept waking her up -- thinking, "oh great, who's guest is this?" and then closed it, locked it, and gone back to bed, figuring she'd ask her roommates about later, if at all. She may even have thought she was overreacting by locking the door. I suspect it was easy to reassure herself that everything was okay when there were so many other people in the house (safety in numbers - it would be absolutely nonsensical for someone to break in with so many potential witnesses! and it was...), each had a phone, some of them were awake, and she hadn't heard anyone scream, yell for "help," or otherwise try and raise an alarm. (MOO -- and I think her statement that she was frozen "in shock" may have been a projection of how she felt once she knew what he'd actually been doing in the house. If she was truly shocked/scared in the moment, I think, as others have suggested, that she'd have summoned LE much more quickly.)