"If his mother had gotten a job..."
How do you know his mother didn't have a job? Not every job provides you with dental insurance. Trust me, I know from experience.
I agree that not every job provides dental insurance. My own current employment doesn't provide it. I pay for necessary dental bills for myself and my family from income earned at my job. If she had a job, one would think that having a tooth pulled or filled would be near the top of priorities for that income. I have had the pain of a toothache like this boy had, and my mother had a hard time paying for dental work... she got the least help she could to deal with emergencies. When I first started working, dental work was among the things I got done first with my income. This woman could have done the same.
That said, I sympathize with her for her loss, because the loss is as painful to her whether it could have been avoided or not (and perhaps her loss is even more painful knowing that had she properly directed some of her income this tragedy could have been avoided.)