The only thing that I can suggest, is that you get more opinions from different doctors. That's what I try to do, for serious conditions.
You might want to find out more about it too. For example, would part of the hip bone will grow back? I think it depends on how much bone they remove. I don't know a lot about it, but I would think that the more knowledge that you gather about this, the better.
It's good that you are questioning it and that you are concerned for your Mom. I often will go along with a medical procedure that a doctor suggests, but not until I understand it thoroughly.