I was on DepoProvera for two years a while back. Worked well for me. The fact that you don't have to remember to take a pill every day is helpful, especially if you have an irregular schedule.
The downside is some women experience a lot of spotting for a week or two after getting the shot.
You'll know she's not really on it if she actually gets a period every month. Spotting is normal, but Depo prevents actual menstruation.