It is horrible being constipated. I suffered for years and the doctors were useless and unsympathetic. So I did my own research and no longer have a problem. The following worked for me:
Lots of water, including warm water - boil up like making coffee, but don't put the coffee in, you can add a slice of lemon or lime if you like. Or try a Rooibos tea bag.
Avoid smoothies. Avoid sugar - one of the main causes of constipation. Avoid fruit juice - full of sugar.
Exercise will help - so the more the better.
Visit the health shop for psylleum husks and brown linseeds. The linseeds are tasteless but have a lovely crunchy texture and can be added to sandwiches, soups, stews, etc. or eaten by the handful.
Cut out refined foods - like white rice, supermarket bread (even wholemeal), white pasta, etc. Give her lots and lots of vegetables, especially raw ones. Find a good bakery for bread. Buy wholemeal pasta. Make your own pasta sauce - any ready made sauces, meals, etc will be full of sugar. Try making proper porridge for breakfast, or buy Jumbo oats from the health shop, put into bowl uncooked with some fresh fruit cut up and some plain, full fat yoghurt (I like the Greek type). You can also buy a good quality dark chocolate (like Green and Blacks) and grate over the breakfast to make it extra appealing).
Have plastic containers full of celery and carrot sticks in the fridge to snack on, as well as sunflower seeds and plain nuts (not the salted variety). My kids all loved snacking on strips of raw green pepper.
It won't happen overnight, and it will take real effort to cut out sugars - no biscuits, cake, fizzy drinks, dilute, fruit juice, white pasta, pizza, instant foods, etc. Don't forget any food labelled "low fat" is probably full of sugar. Weetabix is terribly refined - so will make constipation worse. Good luck! Have patience - and your daughter will get over this. But do help her to make the dietary changes now - before her problem really gets out of hand! It is a horrible thing to go through and I really feel for her.