I'm thinking of ghosting my best friend of 9 years because I don't trust his girlfriend
Is your best friend also not trustworthy?
Yes, the narcissist asshole has no self-restraint, he couldn't even respect a promise he made to his dead mother.
If you're not ok with your friend's behavior there is no point in continue the friendship. It's better to end it instead of making it more toxic.
You are not getting my point.
I get you how much you distrust your friend. I'm just saying you don't need to be friends with him if you have don't want to.