This happened to my cousin in 1953. She survived the tonsillectomy only to die 3 months later of previously undiagnosed diabetes. By the time the doctor diagnosed her correctly, he gave her too much insulin and she went into shock and died.
What a sad story. I'm sorry for your loss.
I'm sure we know so much more about diabetes now than we did in 1953, but diabetes is not usually a disease that requires an immediate interaction or insulin injection. The number of insulin units a person receives should start smaller, and then the doctor should increase them gradually, if needed.
Of course, the early insulins were derived from animal products, and who knows what else. It's possible your cousin had some kind of allergic reaction.