I believe that issue has been previously decided and the answer is that they don't have that right. It is also not the critical question here. The question is has it been established what her wishes were. Of course at the time she was competent to consider this question the law in Florida did not include feeding tubes as life support machinery.
However there is no Federal Right to do so. In theory a State can write a law mandating forced feeding in cases of suicide, which could be defined within the scope of refusing life prolonging medical treatment...no matter how extreme. It would be upheld most likely by the SC but that's not going to happen...The trend is in the other direction...The direction of personal autonomy in end of life decisions. We speak of the value of life..Many people would prefer death to the apparant "value" of Terri's life.