The nature of her crime is completely irrelevant to the topic at hand. People are either in the care of the state or they aren't and if they are then the state carries the responsibility of guaranteeing their health and well-being. In no situation is the level of care during incarceration ever taken into consideration as a part of sentencing. She received a sentence of life without parole, not life without parole plus shittier health care than the rest of the prison population.
The are two arguments that can be used against this. You can either think:
A. That prisoners (murderers included) should not be entitled to proper medical care
or
B. That SRS should not be considered a medically important procedure
If you argue in favor of A, you are arguing against general human rights protections, including the 8th Amendment (see
Estelle v. Gamble)
If you argue in favor of B, then you are disagreeing with the the entire medical community on the proper treatment for gender dysphoria.