The only defense against deadly force is deadly force.
Even the president's body guards have to operate with that premise.
Therefore, dialing 911 is not a defense against deadly force, it's just a defense against it lasting longer than 5 to 25 minutes (depending on response time).
As to whether they will be convinced or not?
Guns are a very emotional issue for many people, part of that is fear from a lack of understanding that is derived from a lack of experience. Most people won't argue the substantive nature of the argument because they're so caught up in the emotional portion of the argument, so much so that the emotional portion of the argument actually becomes an identity argument: who they are to themselves is based on their belief in the truth of their viewpoint.
You can't argue substantive to somebody on an emotional level. you can barely discuss emotions on a purely emotional level, but once somebody is arguing on an identity level, nothing logical, factual, or any other type of rational will have any effect.
The goal should be to try to get them past the identity level and emotional level, so they can discuss issues on the rational level, the substantive level.
To do this, you'll somehow have to get them that experience they lack so they can overcome the fear and other emotions. Shooting a firearm in a safe enviornment can replace that fear with a thrill and a confidence in understanding. Maybe at that point, they will be able to start moving away from their identity crisis: that gun ownership means you're a bad person (or a criminal, or you will be a bad person in the future because the gun will make you--or give you the opportunity to--do bad things), and all good people strive to stomp out gun ownership.