My grandma was in her late 80's and they perform an endoscope procedure on her. I think if they have a good anesthisiologist, and a good gastrointestinal surgeon she should be good to go. Maybe you need a second opinion to see if they can perform the colonoscopy or sigmoidoscopy. They must not be to worry if they can't find a solution. Hopefully they won't plan on doing those procedures when she's too weak to handle them
So sorry your going through this, but I agree with Gayle they have to find a way to stop the bleeding.