Factory Reset the device.

 

That's pretty odd. One possible explanation where this would be "normal" behaviour is if an Instagram activity was opened by the Facebook app when your wife chose to do something (could be something like view photos with Instagram maybe? I haven't used Instagram or Facebook for many months).

Try pressing the back button when this happens, to see if the Instagram activity closes and goes back into the Facebook app's activity.

Since it works fine when your wife reboots the phone (all apps are closed), so Facebook would start up with it's own default activity, and hence works fine.

I am not sure if the above explanation is correct, but you can try.