My mom had both her knees replaced at the same time. The people at rehab thought she was NUTS, but she just wanted it over with.

Rehab was a bitch (But my mom was also pretty heavy at the time, which made it harder), but she has two new bionic knees now, and feels about a million times better and is much happier and pain free, so tell your mom that the long term is definitely worth it, and good luck to her!