I would mention it to my friend. I had a similar situation with a friend in the apartment building I lived in. She had just made a new friend with another woman who had acted strangely with me. I told my friend to "be careful" and told her about the situation that had concerned me with this other woman. My friend didn't think anything of it and continued to be very close with the other woman and the woman committed suicide in a horrible fashion and my friend was extremely shocked as she hadn't seen anything indicating anything "wrong".

You might lightly try to tell a little of your experience if your friend mentions the other person. Do a, "you know, I use to know X and had some trouble with her behavior," etc.