Is it possible the text to Tennessee about Erin's plans for the day came from Erin's phone but not from Erin?
In most cases, I would think a best friend could tell if someone other than their best friend was texting them, especially if they texted often. I got a text the other day from someone's phone telling me they were sick. When I spoke with her later, I asked if she had had her significant other send the text because the linguistic patterns didn't match. The wording, the cadence and the punctuation were all off. They were grammatically correct, but they were not her. And this is not a person I text every day. Indeed, it was her S.O. (nothing nefarious, she was just too ill to cancel plans herself) People know how their friends talk and text.