"There is only one supreme law of life: the survival of the fittest, with its corollary, the elimation (or, through a hazardous compassion, subjugation) of the unfit."
I dunno dude, you seem to have fallen on a slipperly slope here by misapplying the leading theories of evolution which have been vastly reconfigured since the old fish to modern man diagram we are so familiar with. Thing of the past. There is no orderly march forward as a species and never has been. And now you are bogged down in defending mythos.
Survival is all about environment and adaptations to that environment. In our present environment, we have medical machinery of all kinds to help us breath, beat our hearts, clean our kidneys, act as prosthetics, etc. If a person can survive in this environment they are "fit" enough to qualify. Kind of like early humankind used only certain rocks to make tooks because the other stuff wasn't up to the task. Consider our machines in the same light you would consider tools. They are made to modify our environment into things like, you know, agriculture, civilization, etc.
Our present environment also includes a small little thing called "medical insurance" and thus, if this person has some or access to some that will help her run her machines and she qualifies to use it, then she is "fit" enough for our environment.
So really, the only environment she is not "fit" to live in, is one you have created with your mythos. This is a typical problem and is solvable with a bit of intellectual shifting. Since her appearance and reports of her surgeries and pain have touched a nerve with you, you have mistakenly attributed your weakness for hers. You think she is weak because you would not be able to live with her afflications when in fact she is strong to survive in our modern environment whereas you would - I am guessing - likely kill yourself. In this scenario 'survival of the fittest' works perfectly fine.