I am talking as somebody who had this surgery.
It will be helpful to be as fit overall as she can be, also from a cardiovascular standpoint. The pain after the surgery can be a lot but moving afterwards is a must to promote healing.
The recovery also includes the healing of internal ligaments which will be attached to different places. Thus she will be out of weight-lifting commission for about 6 weeks and should only do cardiovascular exercises during that time.
The overall recovery will depend on many factors, not least of which the type of surgery that will be performed.
Since she will be less active for a while and lying and sitting more than usual, I would do as many strengthening exercises for the lower back as possible, including back extensions.