Thank you for your query.Post nasal drip
is a part and parcel of upper respiratory tract
infection (URTI). It is very common for children of your daughter's age to have frequent URTI as they develop their immunity while growing and most of the time they are viral in origin.
I would suggest that you follow your doctor's advice rather than your friend's. Flixonase nasal drops shall be helpful if your daughter has allergic rhinitis
which is giving rise to the post nasal drip. It shall have no effect of the post nasal drip is due to viral URTI.
I hope I have answered your query to your satisfaction.
Wishing your daughter all the best.
Rajiv K Khandelwal