Adam's father isn't getting the notification sounds on his Android phone. Leo says to make sure the volume rocker is turned up for notifications. It has a separate volume control. Turn it up. Notifications on Samsung Android phones work that way. Press the volume rocker control and hold it down until the volume control settings pop up. Then modify the notification sound settings.
Also, turning off do not disturb wouldn't work because it isn't global: just per contact. Leo says that his father must be muting the individual that he's getting messages from and therefore, he's not getting the notification sound. Swiping a message there comes up a gear and a bell, that will mute all messages from that point on, if you accidentally hit the bell. So that's likely the issue.
Adam also says that Team Viewer will let you remotely access another person's phone to fix it.