One of my biggest fears

So I kinda dated a narcissist guy is really good at manipulating and lying for almost a year and it’s been almost 2 years since he blocked me on everything and he’ll stock me on TikTok, call me on unknown numbers. And he even has a girlfriend right now too. Here’s the thing that’s been fearing me, I used to be close to this one cousin who I thought was close but I knew she was a narcissist as well but she randomly stopped reaching out and she had knew About my narcissist guy that I loved. I’ve told her as well it was one of my fears if she ever talked or did anything with him I’ll never forgive her. Are narcissist capable of doing that and why? I’ve been having this romanization and thought about this fear ever since she stopped reaching out to out to me and him as well even tho he still does rarely but it really scares me, idk what I would do if something happened between and I don’t know of im like scared and heartbroken if that would be the case. Help..