Teatree oil in a burner at bed time and morning can help the body.

We use Vicks on chest, back and soles of the feet at night ( for all coughs)

Manuka honey is fab.
Check out the cough meds at your chemist, some thing to sooth it so she doesn't get a sore chest from coughing.
Dry coughs suck because its just there and not much you can do.

Does she have allergies to anything? A lot of spores around at this time of year. They can play havoc with breathing -like pp said, some times an anti histomine can help.
I have mold allergies and spores do me in at certain times of the year.