Most Helpful Girl
If you were to involve yourself with your ex's new relationship, not only will you come off as a deranged stalker you will unequivocally let him know he still has power over you. Which to me means that you're letting him win.
Your attempt to sabotage your ex will only backfire on you. She will come to see him as the victim and more than likely your actions will have the exact opposite effect of your intention. You will drive them closer together because nothing bonds people more than adversity. She will see your actions as an attack on their relationship not as a kindness to her. Which it really isn't because your motivation is revenge not sympathy. In my mind, you're pretty much stooping to his level - you should strive to be above that. Don't allow him to make you into something you despise.
I know how easy it is to let anger take a hold of you, especially if it's due to a deliberate act of cruelty. But this man has already taken up too much of your life - 7 YEARS! - and to allow him to consume you even further is sad. You said you moved on with your life, so my suggestion is to continue just that - moving on.
Karma isn't for your personal gratification, it's the Universe's way of balancing the scale.
Please know, I feel for you, but you are going to have to let this go. Forgive him for your sake and stop giving him prime real estate in your head. You've suffered enough - really.
Best of luck.