For whatever reason, I just don't get the sense that she was much of a drug user. UVA is probably the best school in Virginia. I know all kinds of kids party in college but it sounds like she was a good student with a lot of extracurricular activities. Plus, she went to a high school that is generally joked about having a lot of well to do kids who do a lot of drugs. If this was her cup of tea, she would have been attracted to it in high-school and then probably not have gotten into UVA.
Anyway, her friends certainly know what she did or didn't take intentionally. If she had a lethal mix of something in her system waiting to kick in, she probably couldn't walk for over an hour. And I don't think she had a fifth of whisky at Tempo so I doubt this was an accidental OD that he is covering up. I think they either have their guy or if he's innocent, he's working with LE right now and has convinced them that he dropped her off somewhere else. However, if he's so innocent and was just a friendly ride that night, he would not have hid from LE for 5 days. He would have been like, holy crap!, that's the girl I drove, I feel like maybe I dropped her off in a bad neighborhood and if I tell the cops ASAP, I might be able to help find her. He didn't do that and didn't come forward to say he saw her that night.