Actually, her delicious looking daily menus and foods are all low GL and, by stabilising blood sugar levels other hormonal imbalances do tend to even out. Dr Lara Briden doesn't think hormonal imbalances have much to do with it, but recommends both vitamin B6 and magnesium as 'anti-inflammatory'. However, B6 also helps to balance hormones and magnesium reduces water retention, e.g. breast tenderness, which is a common symptom of PMS.
If PMS was caused primarily by inflammation one would expect omega 3 (which turns into anti-inflammatory prostaglandins) to have a major effect, and omega 6 e.g. from evening primrose oil, which is, if anything pro-inflammatory, but helps to balance hormonal signalling, to have a lesser effect. To date, studies suggest that omega 6 is more helpful than omega 3 and the only study I've seen on omega 3 found expected improvements in mood and anxiety, but not the physical symptoms of PMS.
In truth, what causes what is maybe not so important, I agree with everything Dr Lara Briden recommends and am sure it will help many women. But, for those it doesn't I recommend not considering PMS to be caused by the same thing in all women. In my book Balance Your Hormones I talk about the different kinds of PMS, based on different symptoms, and give a more personalised approach.