Either party can, in theory, go back to court any time they want for any reason they want. If the reason is frivolous or totally without merit, the court may impose sanctions on the offending party. In extreme cases, a judge might restrict a party’s access to the court by requiring a lawyer’s concurrence before additional motions are filed.
Regarding custody, the standard for modification of an existing order is a substantial change of circumstance that affects the welfare of the child. The burden is on the party seeking modification of the existing order to show that such a change has occurred. When such a showing has been made, the court then uses the “best interest of the child” standard when determining whether a change in custody is warranted. In your case, the only valid reason for husband’s ex to go back to court is if a big change has occurred that affects the children and there’s a reasonable possibility this change might convince a judge to change the existing order.
Children in NC do not get to decide who they live with. When a child becomes of “suitable age” - and in NC there is no statutory or common law definition of this age - a judge might choose to take the child’s preferences into account. The judge is not required to do so, however. Even if judge hears the child, the judge is perfectly free to ignore the child’s wishes. On the other hand, there’s really not too much the court can rationally do if a child close to the age of majority chooses to live with one parent or the other.