Why is my friend having terrible leg cramps?

My friend is having a serious problem with leg cramps. She will get one in her calve and the as she is trying to stretch it out she gets one right at the front along her shin. The pain is awful because when both cramps hit (always the same leg) she can not stretch it in eather direction to quicken the relief. I told her to start a multivitamin and magnesium, as she may be depleted of nutrients, but after two weeks she is still getting them and not just at night. We horse back ride together and it is just awefull when she gets one in the saddle. Any ideas on how to help her? which vitamines/minerals besides magnesium help subdue the cramps?