There are various causes of night leg cramps. They can come in association with certain diseases like diabetes, so you might want to get a general check-up, but most of the time they result from a relatively simple electrolyte imbalance.
To correct that, try supplemental magnesium (the first/best thing in most cases), calcium, potassium and sodium, along with plenty of water. Vitamin D may be required at the same time, and in some cases Vitamin B-12 is helpful (the latter because the problem is often actually with the nerves, rather than the muscles themselves). I've heard that Vitamin E can also help, but I have no confirmation of that.
Longer-term, correcting dietary imbalances is advisable. Plenty of raw vegetables and fruits will help, as they naturally supply all of the nutrients mentioned above, whose deficiency can trigger the cramps.
[My wife suffered from these periodically for a few years, some years back, so I had lots of experience being awakened at night by pleas to help "work it out"! She eats better now, and no longer has them.]
