Don't see a ready solution, so he'll probably need to thinker with the N900. Hope some of my ideas can put you, him in the right direction.

Maybe you could look into camkeyd app. How it detects when the camera button is pressed.
From there I could think of many ways, only don't know which one is possible. Depends on how much his girlfriend can use the N900.
Maybe the simplest would be to run the gallery app in the background with the message or the notes app, or something else. Before he gives the N900 to his girlfriend, he starts the camera app himself, and gives it like that. Modify camkeyd to instead of bringing up the dashboard, to search for the other opened app, and bring it to foreground so it's shown instead of the camera app.

If she knows to use the N900, write a script that shows the image, and the modified camkeyd starts that program.

Write a script that shows a similar interface to the camera one, and shows images from the camera, when she presses the button show another picture. This seams the most complicated, though.

If he doesn't know programing, he could ask politely Heikki Holstila (Maintainer) of camkeyd to help him out.
If it's posible to do something like this, maybe I could do it, but it would take some time, because I'm not familiar with these things. And that would postpone the proposal.