I’m so sorry to hear you lost your MIL at such a young age. I don’t know if the doctor told my 88 year old mother she would not survive the surgery.
I had cared for her for a week after she came home from a week in the hospital, and two weeks in a rehabilitation nursing home when she was 78. She had been cut open from her throat to her belly, her ribs spread wide apart for the surgey, and she was still fragile and in pain. At 88 both she and I agreed that she could not/would not subject herself to a similar surgery again. She was far more frail and other systems were also giving out. I think she made the right decision, and I don’t blame a doctor for it.
It was particularly heartbreaking because she had insisted that she was not ready to go, and that she wanted to accomplish more in her life.
I do understand your mother's decision, and it was hers to make under the circumstances. God rest her soul.