This happened to me only once and very recently. A Kappa in my building threw up our hand sign. I was like "huh?" He said that back in the day we used to do that because you all are our sisters. (He's 50+ or so.) I told him that I have never seen that done before.
Soror, now as for your friend -- you've told her but if she does it again I would punch her in the face! (just kidding). Seriously, just like anything else, if you tell someone that something offends you and they continue to do it then that is not a friend.
I would say that maybe she doesn't know any better and no one told her but now that you have.... keep us updated if she persists.