A.,
so much can cause diarrhea. Like the other mom suggested... keep a food diary.
too many fruits or juice can certainly do it.
allergies to milk (even a sensitivity can do it). She may be lactose intolerant like most of my house... try Lactaid - it's regular milk that's been treated to break down the lactose and there's not a single difference in taste (though I can't say the same for price).
She could require more fiber in her diet. If she's not getting enough from whole grains (frankly I think everybody has to work at this... I don't naturally choose whole grain and it's an effort to reach for it instead of white bread, flour tortillas, plain bagels, etc) you could try adding Benefiber to her foods.
I would also recommend increasing the yogurt in her diet, or adding acidophilus. This will put good beneficial bacteria in her digestive tract - even if it doesn't cure the diarrhea, it's a good thing to have in her diet. My kids take a chewable one every day with breakfast. My 4 mo old had persistant diarrhea as a newborn... i busted open a capsule and put the powder in her bottles and it helped her tremendously to kick the runs, which in turn let me cure the diaper rash - oy!
my mom would feed me Barley, Rice and Apples when I was a kid with diarrhea. her variation on the BRAT diet (bananas, rice, apples and toast) for diarrhea.
It would't be a terrible idea for her to see a specialist if the diet changes don't work. It's possible she has colitis or Crohn's - not extremely likely... but persistent diarrhea (i've read) can lead to malnutrition because the food moves thru the system too quickly to be properly absorbed.
Good luck!