Would the Romans have allowed a Jew to stone a Gentile?
No. Under Roman rule, a Jew could not have stoned a Gentile and escaped punishment; the Romans would not have permitted it. Rome allowed the Jews to handle their own religious matters and execute their own people, but that tolerance did not extend to killing a Gentile.
Les explains that under the Roman government, there was no way a Jew would have gotten away with stoning a Gentile; they would never have gotten away with that. The reason Stephen’s stoning was permitted was that it was a Jewish situation: Rome let the Jews pretty much handle their own religious system. The Jews were permitted to kill their own in the name of their own religion, and Rome did nothing. Rome never lifted a finger against the Jews carrying out their own means of punishment. So the Romans’ toleration applied to Jews punishing Jews, not to a Jew stoning a Gentile.