From the top of my head a few things seem possible:
The simplest way is to handle it as flavor and us it as armor. From what you wrote I have the feeling that this won't satisfy your player.
An other idea that comes to mind is, that with an appropriate Stunt you could allow your player to substitute her Weapons Skill when defending against gun attacks. That seems really nice, depending on how high the Athletics and Weapons Skills are. Wouldn't even be overpowered in my opinion, because a good gunslinger can still get around it easily enough.
The third thing you could try is handling the shield similar to the evocation/block rules. In this case you would have to talk to your group how much of a block the shield would represent. In my opinion this is really tricky, because you wouldn't want to make it to strong. It's just a shield anyhow and there is a reason why you don't see those around anymore these days...
Last, if the shield would be an Item of Power (why do I have to think Captain America?
), then what it can do would be reflected in it's powers. That might be a good idea too...
Edit: Correction on the Block idea ... You could just use the Block rules ... basically that would come down to just flavor though, because you could do the same thing with the weapon you wield in the other hand ...