TBH I think you will look spiteful if you go saying 'me and exDH slept together' she will prob take it as 'your fault' meaning you slept with him simply because he cheated on you and is now with her so your jealous and want her to be cheated on too!
Honestly he sounds like a dog who will eventually get found out!!
If you have to tell her somehow annonymously get the dating site info to her, otherwise it just looks like the spiteful exDW causing trouble!!!
ps Id also be getting a full STD check, who knows who else he has been with?!