Do what I did once with a shady ex...
Since you're alum of the chapter, I would go and "hang out" a bit with them, when you start talking about relationships and whatnot, say how bad this "one guy" treated you and how he used you while cheating on his more serious girlfriend. Of course drop a name..and last name. If she asks about it, casually tell her what happened while acting completely oblivious that it was her boytoy. The pieces will fall in eventually for her and she'll learn much earlier than what she would have being used by him even longer *nods*
~Kayla
--Yes, I did this in a similiar situation and it saved me and the "other woman" a whole lotta heartbreak.