That really depends on the individual. How far post partum is she? Has her doctor OKd her to exercise? What kinds of exercise is she used to? What does she like to do for exercise? What are her goals? Etc.
Yeah, what Angabel said... If she is past the 6 week mark shes no longer considered in the postpartum stage. Which is when most women with uncomplicated pregnancy & delivery are given the "Go Ahead" for exercise! Was it "C section"??? Lots of questions to consider? However, top 2 would be, #1. Is she past 6 weeks? #2. Has she been cleared by the doctor?
Yeah I agree with Angelbel and MikeHankins. Based on what I 've read about motherhood, within six weeks, the uterus will have shrunk considerably. She can do a gentle exercise for pelvic floor and abdominal muscles before her 6th week check up. I think the midwife or a physiotherapist will advise her on how to do these.
Walking is a good form of exercise. It is suggested to be done early in the morning where she can breathe fresh air too. However, it is best to seek her doctor's advise too.