Wikipedia on dysbiosis
If she's had gut problems in the past, it's likely that a dysbiosis is present. It may or may not be completely responsible for the halitosis, it could play a major role though.
The probiotics I recommend are the Essential Formulas Probiotics 12 Plus, available in my online store. I typically recommend a loading dose to begin with, then maintenance as needed, depending on how many cultured foods are present in (or worked into) the diet. PM me if you need more info.
Increasing fluid intake (I'm much more partial to herbal teas than plain water) could also help a lot--a dry mouth is a stinky mouth!
Also, she won't know if dairy is the culprit unless she completely takes it out of her diet for a good while. My wife can totally tell if I've eaten dairy, even if I brush my teeth immediately afterwards.