I think Raoullefere is saying that Cosette is using Steve's bathroom rather than the dorm.
As for Steve getting into a dorm, dorm security is notably lax. If you look even a little like a student (or not, I'm late 40's), someone leaving the dorm will be polite and hold the door for you so you don't have to fumble around for your key card. College students typically have no sense of danger (which explains a LOT of stupid things that happen at college).
And it's why, in her first semester at school, one of my daughters was robbed and beaten at gunpoint in her dorm room. By another student. Who was never caught.
And every time after that, when I went to get her at the dorm, some idiot would still hold the outer door open for me. No questions, no ID, nothing. I frequently read the riot act to them.
There's no such thing as foolproof security when you're trying to secure a building full of fools.