My DH had his left knee replaced on September 29 and did wonderfully! He had his right knee replaced on December 11 and it has caused him much more pain than the left. He said that if he had done the right one first, he's not too sure he would have had the left one done at all.
Today has been the first day that he has not really complained with the pain, but the doc had prescribed him some dilaudid when he went for his 2 week checkup on Monday. That was only for 5 days, so it's about over.
I can't imagine the pain that she is in, but you were correct in advising her to have the replacement. She would have NO chance of having any "quality of life" without it; yes, she's going to have to work at getting back what she has lost over the past week. It's my prayer that she will get over this hurdle and begin recouping her strength and drive to improve.
You need to get over your guilt. She wouldn't have had it without thinking it was the best thing for her. As soon as she's able, prod her on with the appropriate therapy and just be her loving son, the cheerleader.
Again, I will say a prayer for her and for you.