He might have heard her yelling 'rape' but being that it was broad daylight, on a busy thoroughfare, and they were both fully clothed, I wouldn't think much of it.... might video it...
He starts ripping her clothes off, it might pique my attention.... or pulling his pants off... until then, the calls of rape would get my attention, but I would hope I would be sound of mind enough to leave the pistol in the holster for the time being.