Ouch, my 78 yr old mother had this done, last year. Not fun. Make sure she has a good Physical Therapist, or the recovery will go very very slow. I hope she does well.
Best of all possible outcomes to you and the mrs. My 83 yr old father just had his knee replaced after it started locking up and giving him problems while climbing and cutting down trees. He swore he was going to stop tree work about 5 yrs ago....but he's stubborn. He has came through therapy with great results.