The tips were not necessarily from a person, but rather cell, or OnStar data collected by the FBI. As for the rest, I think the texts are a little strange being that from where she left the friends house to her house is about a 15min, maybe 18min drive . The fried text her at 304A. Erica should have been home, even if she went north back to the loop and around to get home. So even if the friend was not paying attention to the time, she would have been near the pond where they found her, and still about 20 min from home when she replied stating she would be home in 5min. Or maybe she didn't thing it was the friends biz.