Advice?

Long story short. In may narcissist ex of 8 years and I broke up because I found out he cheated on me again… days later he starts seeing a new girl, posting her, doing everything we did and they immediately got into a relationship. I then thought it was a great idea to warn her about what he’s done and she didn’t believe me because I know the real him, & I knew that the only reason that he got with her was because he can NEVER BE ALONE. In the first month they start dating, he’s still using excuses to text me and then one day he texted me to say that “he loves me & misses me, & that he’s not happy with her.” I text the girl to let her know and turns out that at the same time he was telling me that, he was still in a relationship with her. Later that night he ends up dumping her… & she tells me that he wasn’t putting his full effort for her. Then days later I end up finding out that they got back together, despite the girl knowing that he was telling me all those things… one month later I come to find out that they broke up AGAIN. He then texted me AGAIN. Like the narcissist person that he is, he started talking shit about her saying that she wanted to see him ALL THE TIME, she wouldn’t let him breathe and that he got tired of her shit. He basically blamed her for the breakup & even said that she was “just for fun” “someone to play with” “a pass time”. But according to her, once again he wasn’t putting his full effort for her. So idk what the truth is. All I ask is that now that this girl made her Instagram public, I’m lurking and I hate it. I noticed she’s posting sad quotes about him & im here surprised how she can want him back or how’d she get so attached. They were only together for two months & they’ve broken up twice. She was also aware of the type of guy he is, that he was reaching out to me behind her back and when they broke up this time again, he reached out to me again. She doesnt know that he’s reached out and she doesn’t know that he told me all those bad things about her either and I won’t tell her. How can I stop lurking?