Rotate reminder in camera
That looks like a job for Tasker:

Profile CameraRotate:


Condition: App → Camera
Task: Notify Sound (Text: "Rotate me, please!", Sound: [select file])


This way, whenever the camera app becomes the active (foreground) app, a notification will appear with the text specified. You can also include a sound (maybe something annoying or shocking, to make the "user in question" really aware). Also, instead of the notification you could chose "play sound", and have a pre-recorded MP3 played saying/shooting something like "Hey, don't forget to rotate the device!"