My boyfriend left me for his mistress, but still sexts me. Can I tell the other woman?
Oh, this is all very messy and bad. First of all, I'm sorry this happened to you. I am also sorry for the next person you date as you will (justifiably) be suspicious of every female in his life who isn't a blood relative. Your ex-boyfriend is a douchebag, no question, and it's cool to be pissed as hell.
This whole thing has drama and mess written ALL OVER IT. My suggestion? Eliminate him and her from your life for your own sanity. The best, most mature thing to do would be to tell your ex-boyfriend to stop immediately, that he made his choice, and if he continues you will politely inform his new girlfriend about his late-night extracurricular activities.