Been there and done that! You havenít ruined her hair if its going to be curly it will grow back curly, quite often a big hair cut can mean that they hair is heavier rather than layered so makes the curl drop, or if the curl is a soft one its just been cut at a point that its not long enough to reform. You can do a number of things, when you shampoo your hair use a curl activating leave in conditioner they normally contain magnesium and are available online and from professional salons, always comb it through with a wide tooth comb to evenly distribute and then gently scrunch your hair in your hand as its drying. If you normally leave it to dry naturally or use a dryer and diffuse on slow speed medium heat and just let your hair sit in the diffuser until itís dry donít move it around.
Hair has memory and it remembers its shape when wet so how you treat it during the drying process is critical to the end result. Use the same leave in conditioner or curl enhancer daily just a light spritz and donít brush your hair, use a wide tooth comb or a tangle teezer style brush and dry as above. ...More >
If you are going to be curly they will come back, but this will help, good luck