Theories
Charlie Mac Has Neuro Disease

Warning This Section Does Contain Spoilers

The text messages between Sarah and the mysterious "N" could possibly be Nicholas Matsoukas, who is a neuroscientist that still resides in Chestnut Falls and could potentially be Charlie Mac's doctor. If Charlie Mac had a neuro disease that was effecting his mind, this could explain the mistakes in the prescriptions that he issued to Susan Li's veterinarian clinic. This would also explain the text messages between Sarah and "N". See those solved text messages here