Hi & good on ya mum for finding out some info!
Apparantly having light periods can be common of pcos can't remember where I read it or why coz I've read so much stuff but i have read it I too have light periods most of the time when I have them.
you should have a period evry three months is whats recommended but can be somewhat contraversial too but most dr's recommend 3 mths.
Your daughters symptoms sound like how mine started mine have now gotten a lot worse but I think thats due to the fact of me gaining so much weight!

Diet & excersise are so important with pcos & I wish I had've paid attention to that lil detail yrs ago cos being bigger has made my symptoms a lot worse.
I don't think it matters what your menses is like coz pcos is so confuzing I've never heard of that being a way to tell if you ovulate??. You can definetly have annovulatory cycles though which mean you don't Ovulate but still have periods.
Metformin Is deffinetly worth a try It can be very benificial to pcos.
I hope the info I have given is helpfull & not confuzing! There are a lot of women that know a lot of stuff so if U have any more questions keep asking!!!