No way to know without knowing her history, or knowing what her oxygen
saturation was during and after surgery.

HMc

You should ask the doctor or nurses that treated her. I suspect that they
conducted several different tests on your sister prior to the laminectory.
Perhaps one of those tests indicated that she needed the oxygen. On the
other hand, perhaps they were trying to make more money from her or her
insurance company. I heard of one case where a certain hospital--don't ask
me to name it for legal reasons--were conducting blood tests on all people
after they died. When one doctor complained that there was no need for it
without a doctor's specific written order--they stopped the practice. It
was obvious to the doctor that complained that the hospital was doing this
to make more money from the insurance companies--they added the cost of
the blood test to the final bills sent to the insurance companies.

http://www.stjohnsmercy.org/healthinfo/test/ortho/TP035.asp

"The anesthesiologist will continuously monitor your heart rate, blood
pressure, breathing, and blood oxygen level during the surgery."

Perhaps she was not as satisfactory as they would have liked, and perhaps
they were trying to err on the side of caution because of her age.  

You have the right to discuss any charges with hospital billing office.

Sorry, one more thing...is/was she a smoker?  That may be a large reason
they gave her O2

The answer she got seems pretty self-explanatory.  I can't imagine what
anyone here could add since they know nothing of her case.