If Mike goes into the app settings, he can make Waze his default navigation app. He'll also want to get rid of Google Maps, or at least disable it. He may need to go into the Google Now settings as well. If he goes into the Waze settings, he can tap on "Open supported links" and select "Open in this app."
This worked for Leo, but Mike is on a Samsung Galaxy S8, so it's conceivable that Samsung has done something to prevent him from being able to change that default.