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.
A minor's standing in the State courts is often dependent upon age so many do have a say so (barring treatment already agreed to with a doctor). Guardians (this includes parents) do have standings in all courts in this land regarding those they have guardianship over.
As for no good argument - There are religious groups who refuse transfusions and other medical treatments as a matter of their religious freedoms. This includes those who will not use contraceptives or abortions according to their doctrine (Catholics) and those who refuse blood transfusions (Jehovah's Witnesses).
Though I do not agree with not using contraceptives (barring abortion inducing) and not using blood transfusions, I maintain they have their rights to do so under the freedoms afforded under the US Constitution and Bill of Rights.
You may not agree with that. But if so you may be seeking a form of socialism or communism that imposes the will of the state over the individual.
Is that the case?