Not being of a religious persuasion, one would wonder why your friend gave it to you. Also, as you are not of religious persuasion you are quite free to do whatever you like with it.
A catholic person may wish to keep the water which has been blessed by a Priest in order to use it to bless themselves or their home and recite a prayer. If I were you I would give it back to your friend and just say thanks, but politely explain that it is really not the type of thing for you and perhaps she could give it to someone who might find an appropriate use for it.
Whilst you could easily chuck it down the drain, drink it or wash out your nostrils with it, there really is no need to be disrespectful about it, so handing it back would seem the most appropriate thing for you to do.