Well a bit of self control wouldn't go amiss... I have PCOS, I've never found it made me feel particularly depressed or moody (I'm naturally moody!) but around the time my period is due I feel a bit subdued and grumpy but I generally try and keep it to myself, I think if you know you're going to feel a certain way then it's best to exercise a little bit of control and just remember it's only a moment in time, I also try and make sure that my reactions to things that annoy during that time of the month are appropriate (as opposed to all other times when I'm just mardy!). However in my experience, excercise helps a lot for both depression and mood swings and it would be worth her trying that out.
I think there's some information that says people with PCOS should try and watch their carb in-take as well (which is annoying as it's my favourite food group) and certainly I can feel a bit sleepy if I eat too many carbs (suspect everybody does though) so if eating too many carbs and then feeling very sleepy then it won't help her mood much. A good diet will help your daughter, especially as PCOS can mean you might be more inclined to put weight on.
Also, when I was diagnosed with it I took dianette which is a very mild pill and I can endorse it. The only reason I came off it was because I wasn't in a relationship and since my PCOS can be controlled with diet and exercise, I didn't feel the need to be on it.