You know, that sounds tricky. If you try to AMOG her new bf using his age, for example, you'll actually be sounding like a jealous jerk and DLV yourself.
Don't sarge her. Sarge her new boyfriend instead. Be friendly to the guy, and work your way to have him trust you (remember, you still ARE the previous boyfriend). Enjoy yourself. Neglect her. No hard negs or anything, be friendly to her, but neglect her. Let her see you having fun and not giving a shit about her.
Whatever you do, do not sarge her directly, ESPECIALLY if she starts flirting you. If she does, it will be a test. Be smarter than her.
As a bonus, if your friends circle sees how well you handle the break-up, you'll gain social proof. Very important
