Today is the yahrzeit of the Ohr HaChaim haKadosh, so let me mention a question he raises on the parsha. We know why Zimri was killed by Pinchas, but why was Kozbi deserving of death? What issur is there for a bas noach to have relations with a Jewish man? There is an issur for a bas noach who is married to commit adultery, but who says Kozbi was married? And even if she was, how would Pinchas have known that? Safeik nefasos l'hakeil!
The Ohr HaChaim suggests that just as we kill an animal which a person has relations with so that the sin is blotted out and people don't see the animal and comment, "Oh, that's the cow Ploni sinned with," so too there was an obligation to kill Kozbi because of Zomri's sin.
Other Achronim suggest that the issur here is one of lifnei iver, as even a ben noach is not permitted to abet even another ben noach in sin.
I had in mind as follows: Rashi (Baba Basra 50b) writes that the halacha of safeik nefasos l'hakeil is based on the pasuk of "v'hitzilu ha'eidah." Tosfos disagrees, and argues that no pasuk is needed -- logically, a person cannot be killed unless we are certain of their guilt; to act otherwise is tanatamount to murder.
Is it possible that if the din of safeik nefashos l'hakeil is based on a special pasuk that it only applies to Jews, but not to bnei noach?
This is not a great answer because: 1) Tosfos is clearly compelling; Rashi must have some other reason for introducing the pasuk other than to tell us that you cannot carry out a death penalty in a situation of safeik (maybe more on that in the future...); 2) Safeik nefashos l'hakeil is a rule that tells us how beis din should function; who the defendant is (Jew or ben noach) should not make a difference.