"Big Keyboard" android app has taken over

My Dad installed the "Big Keyboard" app, in order to be able to read things on his screen better. But it's virtually taken over his phone, affecting everything with super-large icons/words to the point where it almost resembles malware. I went into the apps settings and found the program, but the "Uninstall" button is grayed out (of course). I thought that perhaps the program had made itself a device administrator in security, but after checking device administration, it's not listed. So at this point, I don't know what to do short of factory-defaulting the device.