I saw what happened with my cousin. It was the most horrible thing that can happen to parents, IMHO.
It reached a point where they could have done more radiation but the effects were causing such unbelievable pain for her that they had to make a decision to continue, with indeterminate chance for success, or let nature take its course. After many hours of research and consultation and many more hours of agonizing, plus many, many sleepless nights they decided to forgo more therapy. This situation was not unlike what the family in Virginia is going through, except that they believe they have found an alternate therapy which will cure their son. My cousin didn't find any alternates that they believed would give them hope.
I was thinking about what would have happened in my cousin's situation if someone had gotten the authorities involved because they disagreed with the decision to discontinue therapy. The family was already traumatized by what they had been through, then to have someone step in and take away their chance to decide what they all believed was the best course of action would have been heaping even more abuse on them. I can't imagine having to make the choice to see your daughter living every day in agony or letting her die a peaceful death. But I am certain that the family would not have wanted someone else to make the decision for them.
The boy in the Virginia case was apparently in much better shape, with a consensus of mainstream doctors believing that he had a good chance of a complete cure with the treatment they were recommending. I haven't heard of any cases where courts got involved in ordering treatment for a minor who was in an advanced stage of a fatal illness and the recommended treatment offered little chance of a long-term cure. There probably have been such attempts here and there, but they probably don't get far, because there are plenty of mainstream doctors who will testify in favor of the no-treatment choice. And in the case of a child who's legally a minor, but clearly old enough and competent to have his/her wishes factored into the decision, I think it's often appropriate for a court to get involved and make sure that loony beliefs of the parents aren't being imposed on a near-adult child who doesn't really share those beliefs. In the Virginia case, a court heard about a 16 year old boy whose parents had named him "Starchild" and were refusing standard medical care for a fatal condition, and instead taking him to some sketchy alternative clinic in Mexico, and decided it had better get involved and check things out.
It's often hard for a child -- even a teenager -- to voice opposition to the parents' choices. The parents have usually been the day-to-day caretakers of the very sick child, creating a sense of loyalty in the child, and have also had tremendous opportunity to "brainwash" the child into their own views, no matter how loony. Thus a child who, for example, has been drilled from birth in how evil it is to eat any animal product, even dairy that doesn't involve killing the animal, is likely to say s/he supports his/her parents decision to continue that dietary program, even when doctors say it is causing severe illness and permanent damage, and even when the child is really so sure that it wouldn't be a good idea to try eating what the doctors recommend.
Is it infringing on the rights of the parents for a court to intervene there? Well, yes. We start with a basic assumption that parents should be allowed to raise their children as they see fit, and these parents hold very dear their belief that eating animal products is evil and unacceptable. But it would be infringing on the natural rights of the child for a court NOT to intervene. And if court interevention is ever acceptable, then we also have to accept that there will be cases where a court sees reasonable cause to take formal action in order to fully investigate a situation, and then ultimately finds that the parents' choices are within reason and should be allowed to stand.
I'm glad that your cousin's family wasn't subject to any court intervention, but as I noted, I think it would be very unusual in a case such as you described (as it should be).