This is a tough one. While you certainly have the right to expect that your workplace be a neutral zone, your co-worker also has the right to bring her friends to work.

I seriously doubt that talking to your co-worker will result in anything other then her running back to your ex-BF to report every word you said, so I wouldn't bother.

Unfortunately, your only real choice is to breathe deep and focus on managing your anxiety if she does appear and absolutely refuse to engage with her in any way.

This will take some guts. It is extremely difficult for women, especially, to be rude. But you have every right to literally turn your back on her and walk away if she tries to approach you.

You can also hold up your hand to protect your space and say "Don't even try," before walking away. The one thing you cannot do is reply to anything she says, because that is what she's looking for. Don't let yourself get sucked in.

Also, keep in mind that you have done nothing wrong. She is a vicious child who does not deserve one second of your time.

I hope that helps.