I just read a thread from someonw saying "my friend smells bad how do i tell him" and everyone said be honest and tell him, now we get almost the opposite side "my firend is so rude as to tell me my hair is greasy" have you considered maybe your hair is greasy? i know if i wash mine everyday it makes it more greasy and i have naturally quite thick hair anyway so it looks greasy i cant help that, some tips though are
change your shampoo, don't always use the same one your hair "gets used to it" switched , i tend to alternate between 3brands of shampoo everytime i wash.
dont wash everyday, wash every 2-3days
and ask your friend how she stops hers form being greasy tell her you washed it and maybe its just the lightingin the room/texture of your hair/colour that makes it look that way,
try see it from her side instead of presuming she is just being nasty
if you find out she is just being nasty take everyone elses advice, but check first she isnt just repsonding to a "my mate has greasy hair people laugh at her how do i let her know" scenario