I'm betting that the shot that she went on was Depo-Proveria?
If she is, there's a thread here:
http://www.tfproject.org/tfp/showthr...highlight=Depo
What has been suggested in other places is maybe she might need to get on another form of birth control if she is on Depo as it has been reported that it lowers sex drive. If she does end up switching her birth control, take note though that others have said it took quite awhile for her to get back to where she was before hand.
Maybe have her talk with people at Planned Parenthood or wherever she get's her shot, and see what they suggest, as I suspect they've heard any question that you could think of.
Good luck, and let us know how it goes.