Perhaps you can skip the "before" photo, but it may help to reduce double doses.
If the contrast is high and you can calibrate the app using photos of an empty pillbox somehow, it doesn't look imposible. On the other hand https://xkcd.com/1425/
* The app has an alarm (like a clock). You can configure the time, let's say 9am.
* When it rings, the app put a message in the screen "Time to take your medicine. Please pick the pillbox and take a photo".
If he doesn't take the photo, make the app sound and show the message again after 2 minutes. After a few tries, send a message to some relative that is the guardian.
It will need some calibration to recognize the pillbox.
* After the photo of the pillbox, show a message: "Great. Today is Monday. Open the Monday slot and take another photo".
Repeat if there is no photo, and send a message to the guardian after some time. I'm not sure if in case of failure it is better to go to the previos step or not.
* After the photo of the open pillbox, show a message: "Take your medicine and then and take another photo"
It will need some calibration to distinguish the pills and the empty slot.
* After the photo of the empty pillbox, show a message: "Close the pillbox and store it. Great. See you tomorrow."
Something like this will need a lot of testing and perhaps FDA approval, but it doesn't look impossible.
(Perhaps it is easier to start with contraceptives pills, that have younger users that are more use to apps and don't have memory problems.)