The very MINUTE she turns 18, she is free to go anywhere she wants. Of course, as free9man says, she may not be able to bring her son with her, but SHE can move to any state that pleases her - when she is a legal adult.
As long as she is still 17, there is no guarantee that she can leave without repercussions.
And what's wrong with your 30 year old son, anyway, that he finds equal companionship in a 17 year old who lives in another state and has already shown questionable judgement in her life?

