Hi,
Welcome to Healthcare magic.
You have not mentioned how old she is and about the tests that have been done.
There is no specific test to diagnose IBS-D.
Before attributing the symptoms to IBS-D, other conditions causing these symptoms like infection, malabsorption, medical conditions should be ruled out.
A complete blood count, stool test, blood glucose test, upper GI endoscopy with duodenal biopsy, colonoscopy should be done.
if it is IBS -D, then probiotics, antispasmodics and antidepressants may be given.

IBS is a disorder that affects all ages, although most patients have their first symptoms before age 45. According to the current IBS diagnostic criteria, abdominal pain or discomfort is a prerequisite clinical feature of IBS.Reassurance and careful explanation of the functional nature of the disorder and of how to avoid obvious food precipitants are important first steps in patient counseling and dietary change. Occasionally, a meticulous dietary history may reveal substances (such as coffee, disaccharides, legumes, and cabbage) that aggravate symptoms. Excessive fructose and artificial sweeteners, such as sorbitol or mannitol, may cause diarrhea, bloating, cramping or flatulence. As a therapeutic trial, patients should be encouraged to eliminate any foodstuffs that appear to produce symptoms. However patients should avoid nutritionally depleted diets.

There are treatment options available. It needs evaluation of complaints and disease condition and then prescription of appropriate treatment.