I've never seen the film so can't comment on her knowledge of guns. I will say that I'm from the UK and we don't have guns at all, but I do know how they work. And that they are very basic and simple. Hell, today's guns are just fancy flintlocks đ¤Ł.
I'm, in principle, with OP on this. From looking at that device around her head it does look like it'd be easy to foil, it looks like there's quite a gap between the pin and shotgun casing. Stuffing a towel between them wouldn't take a long time to do.
However, with that said, I will also say that if I were subjected to that experience I would be terrified and so probably wouldn't be looking at ways to disarm it, but to follow the instructions and hope I got out alive.