Is it possible she has a chronic bladder infection? Sometimes there are no overt symptoms such as burning. Frequent urination may be a good thing, because it prevents backflow and pressure on her kidneys, something that people with spina bifida must be careful of. See the following posts:
http://www.healthcentral.com/incontinence/c/8257/8838/juice
http://www.healthcentral.com/incontinence/find-drug-122489-73.html
Hope this helps.
~Pete