So, I mentioned finding videos and putting them out there for tactical review. In almost every scenario I find there is opportunity to get your gun into play. Most low-life criminals don't have much of an inkling of how to handle a gun even though they use one in their "job". However, in this situation, I'm going to say you're pretty much hosed. I put myself in the employee's shoes (though I'm not going to say I'd be at an adult store, but that's another issue).
Not a great way to start my series of tactical reviews, but a great example of when NOT to try something. The problem with news is they edit video. However, from what I can see the guy walks in with shotgun leveled at the employee and never waivers in having the muzzle cover him. The only time it is taken away is when he turns to leave. Eh, that's legally iffy in most places. I guess at that time I would draw and just make sure if the guy turned or came back I would be at the ready.
Other thoughts? I'm going to say if I'm constantly staring at the muzzle of a shotty, I'm leaving well enough alone unless I see action that makes me think the situation might get worse. The only other thing I could think would be to look behind him repeatedly to make him think there was something needing to draw his attention from me, then make a go of it.