I've seen security guards stand and watch as shoplifters exit with items. The guards do nothing. They might even be ordered to do nothing. If they intervene, the store might get sued.
Security guards don't enjoy the same immunity that police do. Their main purpose is to act as a deterrent by their very presence. Watch and report.
In this case, the security guard should have said something. He should have told the masked people to let the Jewish student enter.