It's very difficult to give useful advice when you can't see how the person is blowing...

To obtain the best sound the 'embouchure', or hole in the lips, needs to be very small and aimed AT the opposite edge of the 'sound hole', where the air stream is split to produce the sound.

A slightly silly grin is needed to obtain the required shape of hole, but it takes a little time with a lot of trial and error to get it right.

Many flautists 'kiss' the hole to position their lips before rotating the instrument a little and blowing.

The good news is... once you find the right way to do it, it quickly becomes second nature.

Do warn her not to keep blowing if she's getting a headache - that is hyperventilation, which means she needs to rest!