פירוש על עבודה זרה 36:8

There is indeed a dispute among tannaim about whether one may go to a stadium. The first opinion holds that despite the fact that he might do some good there, it remains prohibited. But R. Natan allows a Jew to go to the stadium because he might have the opportunity to save the victim and even if he cannot, he will be able to serve as a witness to allow the man’s wife to remarry.