Most Helpful Guy
It's not your business, as he's not your boyfriend anymore. This is a good way to stir up some childish chick drama if you go getting involved in this.
And I'm not sure from your question but if I read it right, you don't even know for sure if she's cheating on him...only that she "cheats on every boyfriend she's had". Regardless though, still stay out of it.
Also, +1 for proper use of "she's". Most youngin's are dummies when it comes to grammar lol