Tell your girlfriend it could be any one of several things. Some of us just have vaginal environments more conducive to yeast infections, but there are other reasons someone might get them frequently.
I've had this problem and the best way I've discovered to beat it, is to start by getting a prescription of diflucan, a one-pill cure. If she can't for some reason get to a doctor fast enough, a seven-day course of monistat will probably work. My doctor told me that the one and three day courses really do not work too often.
After that, start taking pro-biotic pills, which are live bacillus culture pills that discourage yeast growth in your body, and drink a lot of cranberry juice, which has a similar effect. She should make sure and get the sugar-free kind because sugar has the opposite effect. If she does this religiously, she should start to see a reduction of yeast infections.
Some of us are more prone to yeast around PMS time, and the above cure should help with that too. However, frequent yeast can be a sign of other things, including (don't freak) HIV. It's very rare for a male partner to be passing it along, but not totally out of the question. So if the simple solution I've suggested doesn't reduce the frequency of a yeast infections, your friend really needs to see a doctor who is willing to investigate a little.
It's all yucky, isn't it? Good luck to you.