And I can't believe that the Resident Assistant (RA), Diamond, didn't boot him out or escalate the issue to Student Affairs or Housing if he refused to leave. My heart goes out to her because she obviously feels some guilt over Lauren's death (and the Mank was so very sweet, trying to make her feel better), but it really is part of the RA's job to make sure that only paying customers (so to speak) are living in the dorms. In retrospect, that would have been one way to get rid of the guy--let someone know he wasn't paying to stay there! Money talks, even if Campus Security is sitting around with their thumbs up their ass.
To partially answer the second question, Lauren met Sean (I forget his real name, too) at a bar; I believe he was working Security.