Baclofen is a strange drug. Some people cannot tolerate it.
It is hard to discontinue off of.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1121408/

I would really consider using a magnesium supplement. This can relax muscles considerably. And low magnesium may be the CAUSE of muscle spasms! One has to have good renal function to use magnesium supplements however. But this can be considered and may work. If your mother uses diuretics for blood pressure, these drugs deplete magnesium! Also diabetics become low and the drugs used for reducing acid in the stomach, diminish magnesium absorption from food.

Magnesium is used by OBs for severe Braxton Hicks contractions during later pregnancy for women. It works very well. Estimates are that about 70% of Americans do not get the RDA of magnesium any more from diet.

I have a magnesium thread here:
http://neurotalk.psychcentral.com/thread1138.html

You want to make sure your mother is not having a Cauda Equina event from spinal stenosis. This leads to incontinence too and severe back pain. It is considered a surgical emergency.
http://emedicine.medscape.com/article/791613-overview