You should tell her you used to know that guy, and he has HIV.

Doesn’t have to be true, but fuck her! I mean, not with your penis, because ew, but just in general. To hell with her! Gaslight the fuck out of her, and give her anxiety and crippling depression. Act like you’re being a supportive ex, while at the same time driving her to the depths of insanity.

Bonus points if you can convince her that the guy actually died 10 years ago, and she slept with a ghost.