Declining care would not be illegal in the US. You didn’t specify where they live, so I can only give that.

For a person not the patient to decline care, that means care was offered or suggested by a doctor or other provider.

When a patient is unable to make their own medical decisions, that task falls to someone with the ability to make that decision legally. Exactly who gets that task varies by location, but if the patient has a healthcare power of attorney, they can name whoever they want. Otherwise, it tends to end up being a family member, and the children of the patient are going to be the first ones in line.

Neglect would require that the person acting on behalf of the patient did not seek medical opinion when doing so would be indicated. As an example, if the patient falls and breaks a hip, that requires treatment, so would possibly be neglect if their guardian/poa/conservator/other did not act accordingly.

But, once in the hospital, that same person could choose to decline care of pretty much any care, though a doctor or hospital has recourse if they feel the decision is outside of reason. Typically, for something acute like a broken hip, you get the hip fixed because there’s not a good reason not to.

But something like chemo for terminal cancer, or refusing extreme care in the case of other terminal events, that’s almost always going to be in the hands of whoever has the authority to make the decision, and would not be a crime.

There’s a lot of vagueness to this because criminal neglect is a case specific thing.

You didn’t provide much to go on, but the child of a person making a medical decision about end of life care is normal. But that assumes the person can’t make their own decisions for whatever reason.

Edit: this stuff can vary from state to state in the US, but it doesn’t vary so much as to be a complete difference.