PCOS diet. We don't know for sure (there are no large-scale studies) but a moderately low-carb diet like weight watchers or south beach should be good, with more complex carbs like whole grains and less sugars and simple carbs - higher protein and lower glycemic-index than a low-calorie low-fat diet. We don't know if extreme low-carb diets like atkins are safe or effective long-term.
Answer for Crohn's. Diet for crohn's largely depends on what part of the gut is affected, how active the disease may be, & has the gut been surgically altered (e.g. "short gut"). When actively inflamed, many patients experience problems with lactose, gut stimulants (like caffeine), & a lot of dietary fats or fiber. Consideration should be given to appropriate vitamin & mineral supplements, esp. If ileum is involved.