Evaporative emissions control leak, small. What I've done so far is inspect the cap -- it looks fine, and I noticed that that little metal flapper valve that covers the fuel inlet is missing (the one that pushes down when you insert the filler nozzle). Could that cause the code, and if so, how easy is it to replace?

It's my mom's car so I'd like to steer her right if possible.

 

Flapper missing won't cause the code. I would just leave it alone.

9 out of 10 evap leak codes will be the gas cap.
Although it looks ok, I'd replace it anyway. ($20)
Just make sure there is no rust build-up around where it seals.