Not ASPD but have two stories to offer you.
I’ve been married to a sociopath for over a decade and I was devastated when I put the pieces together. My closest friends are his closest friends because that’s how it works a lot of the time shrug. I told my three best friends who make up our core group, all of whom have experience with others like him. It suddenly made sense to all of us. My husband wasn’t surprised when I finally put it together, was rather relieved, and doesn’t mind that these friends know because we’re loyal to a level that most people can’t comprehend. We accept it about him and love him anyway; all of us knowing was less of a danger to him than us spinning, trying to figure out why he would do hurtful shit. Frankly, it made his life better for us to know, because we aren’t the type of people to run. Someone (not the person mentioned below) recently threatened me online and I laughed out loud; he asked what I was laughing about and I told him that my first thought wasn’t fear, but that they should watch what they said because my sociopath husband would kill them and not feel bad about it. He shrugged and was like, yeah. I’d never publicly out him, though.
On the flip side, I recently and very, very publicly outed someone’s behavior, but stopped short of labeling them. I’m not professionally qualified to do so. I don’t feel bad about it; the things they did were horrible. I’m not talking about “normal” shitty/manipulative behavior, I mean traumatized children and lives ruined. I exposed the lies and left it at that. I’m also several states away, live with a sociopath, and am not in any danger by doing so. Would I do it to a “garden variety” sociopath? No. This was extreme. In any other circumstance, I would have said it’s not my business and walked away, but this was extremely close to home. I’ve played in the cluster B sandbox my whole life and weighed the cost/benefit carefully. It doesn’t sound like you have the same level of...experience?...with this.
Pick your poison but it’s unlikely you’re as well-insulated as I am and if you’re right, you’ll likely get some unpleasant consequences. And unless you’re their therapist, it’s not really your place to label them something you have at best anecdotal proof of; if you are, you can’t tell anyone anyway. If you feel uncomfortable having a front row seat to what you feel certain will be the destruction of someone’s world, your best bet is to quietly cut ties and call it a day.