Narcisstic Ex moving on

I dated a hot as hell, crazy as shit guy who made me feel like I was on a roller coaster ride of extreme highs and lows. I was addicted to those highs and put up with the lows. I noticed he had a new friend on Facebook in June, when we were still dating. I also noticed that he liked her profile pic when he didn’t do the same for mine. We broke up in September, and I just saw they are now Facebook official (with a relationship start before ours ended). I am going to keep silent - but I am sooo tempted to message her to warn her of his cheating and narcissistic ways. Ugh!! Just needed to vent!