Try booting the phone into safe mode, which doesn't start third party apps. Then he should have access to his app manager. You'll want to either Google how to do this or post the phone model here so someone can give specific instructions. There are multiple ways to boot into safe mode depending on the phone and I'm not familiar with all of them.
If this app was downloaded from a place like Google Play, he could also try going to the app's page and uninstalling from there.