No one can make diagnoses through internet, so we don't know the whole picture here.
There are tons of reasons of why your friend's doctor doesn't think the surgery is an option for her. For starters, she's already fibrillating, which is a risk under anesthesia, she might code on the table.
Colonoscopies are also quite risky procedures in people her age. Colonoscopies can cause severe dehydration even if you are hospitalised, as well as causing tachycardia, which in her current heart condition could kill her.
Every doctor considers pros and cons for each patient. If your friend has doubts she should talk to her doctor about them.