What I watch out for is when the courts try to legislate treatment plans not agreed to by the guardians. Our increasingly Socialized society is reaching out for those powers and will seek to dismantle parents or even guardian's right to be involved in the child's life.
Her disease is mostly curable and treatment can be considered life saving.
A minor cannot refuse life saving treatment and in this case I don’t see a good argument for letting a guardian do so in her place.