Not a lot of point going after her. She has to live with knowing, had she done things differently, Rowan would probably still be alive and her only child would not be facing a first degree murder charge. And that each & every one of the 122 people who live in Stella know that.
Self guilt is never a defense, nor is it a reason to not prosecute someone. Since they're apparently not going after her, it tells me they don't have enough to convict, or she cooperated with the authorities and (big perhaps) won herself some consideration. Or, she really wasn't a part of the situation, other than unwisely allowing her vehicle to be used without learning why beforehand.