My mom had it done and said that she wished she hadn't waited so long. Is it painful? Absolutely. Is it worth it? Absolutely.
But, our opinions are just that and worth about as much. She needs to sit down with her doctor and tell him what she wants to do afterward, what her expectations and hopes for the future are. Then she needs to listen to what he has to say and realize the reality of her situation. There may be some activities that she has to adjust, modify or quit all together but she'll have to do that whether she has the surgery or not if her knees are that bad.