Should I tell my friend "no"?

I recently went through a bad breakup with my girlfriend of three and a half years. She basically cheated on me (even though she says she didn't), told me she wanted a break with no dating, then went on a date (telling me it wasn't one...plus she'd started dating him before the break), then finally told me the truth and left me for the guy. The guy is 7 years older than her and has taught her martial arts since middle school. I went home for support through this, and subsequently a lot of people know the story. Now my ex, who wants to remain friends (possibly sometime in the future for me), is worried that word will get around to her parents in our hometown. We are both in college, she still lives at home when she's not at school, and is afraid her parents won't let her come back home or buy her the car she wants if they know. (The older guy also still lives with his parents and hasn't told them either because of the age as well). My friend wants to send an anonymous letter to her parents to let them know the situation. I think it'd be hilarious of course if this happened (as she treated me like crap through this whole ordeal and the karma would serve her right) but also don't really want to get in the middle of it. Should I encourage my friend not to send the letter, or let it happen and possibly help him? Thoughts please.