What's the etiquette for telling a sociopath you know he/she's a sociopath and you're cool with that?
I agree with TK.
You don't need to tell them; if you are okay with that, just act like that.
Demonstrate it, but without too much effort, understand? Be natural. This is what they mostly want. They are people just like you, nothing less nothing more that for sure don't like to be considered as different, and thence they usually don't want you to behave like you're sorry for them or whatnot.