If she's having systemic yeast problems, it may take a lot more than external treatment to get rid of it....it may take a complete revamping of her diet, cutting out white flour, white sugar, soda, fruit juices, processed foods, all the crap that yeast thrives on. She should definitely see her doctor and get bacterial vaginosis or other infections ruled out. If her yeast is really bad one day's worth of Diflucan may not cut the mustard...when nursing moms get systemic yeast the recommendation is two weeks' worth.
And yes, her partner should get treated too. Keep in mind though that unlike, say, chlamydia, a yeast infection is not a sexually transmitted disease per se - it's an overgrowth of an organism that lives on all of us, all the time, and takes hold when things get out of whack. So there shouldn't be any blame or shame attached to having yeast infections (or other STDs for that matter, the only blame or shame is not practicing safe sex and/or getting them treated! a disease is not a moral issue). And he definitely should not get circed because of it - any more than she should cut off her labia to reduce the areas where yeast could thrive!