If she thinks teeth that bad are just going to fix themselves, what about other possible injuries? Will she think a deep cut isn't too bad because it doesn't bleed much. Will she think a painful arm isn't broken because it doesn't look broken?

The child was complaining about toothaches and headaches for more than a year! To me, that indicates that she doesn't much care what the child feels or his well-being.

I would not be adverse if he were returned after his mother underwent some serious education including parenting classes and showed major improvement.