How do I talk about my abusive former advisor if people ask?
First, let me say that I'm sorry to hear that your relationship with your original advisor has deteriorated to such a state that you have been emotionally scarred by the relationship. That is certainly not the desired outcome.
That said, you have a very important cautionary story to tell. Unlike cactus_pardner's answer, I don't think you should &quot;jokingly defer,&quot; but instead take a very serious tone about the situation:

I wish I could be more positive about my relationship with Prof. X.
If you are serious about working with Prof. X, then there are some things that you should be aware of.
This may not happen to you, but I had some significant issues during my time working in that lab.

You can then offer to talk with the asker in more detail, if she so wishes, and leave the choice up to the asker. But do not blow off the discussion. Hiding the situation will just allow it  to continue unchecked, which is exactly what you do not want to see happen in the long run.