I believe that this IS a federal issue - the right to LIFE, liberty and pursuit of happiness. I have no problem with them steepping in. Now if they would do so on the 1st, 2nd amendments and property rights.
It's also a right to self-determination.
You can debate whether or not she really wanted this, but that is the only question that counts.
Years ago, after Cruzan, the US Supreme Court gave states the specific right to decide whether or not to use clear and convincing oral statements as though they were written out living wills.
Florida decided to allow just that. That is why we are here. Not because they don't think she has a right to live, but because she has a right to resist medical treatment, including feeding tubes.