There has to be something that can help her. Due to her age, I'd take her to her doctor and go by his/her advice. Medications sometimes require lower dosages in regards to the elderly. Some OTC drugs may be simply to strong for her. Again, take her to the doctor. Good Luck,
One simple thing to try, and it's harmless, is having her drink 4 to 6 ounces of tonic water (with quinine.) My dad suffered with bad cramps and this helped him. Also bananas and orange juice both help as they are high in potassium. Not knowing your mom's health history, these aremthings that may bring her some relief while waiting for a doctor's appointment. But she should see her doctor as sometimes having too high or too low a potassium can be a sign of something else and sometimes needs a prescription.
Honestly, the only thing that has helped me is the following: significant dose of potassium supplement and herbal tablets of 'cramp bark' from Health Food Store along with light/gently leg stretches before bed and as soon as I feel or are awakened by a cramp.