IPhone won't always show notifications from Hangouts?

Asking for a friend -- he has an iPhone and has occasionally told me that he doesn't always receive notifications of texts from Hangouts from me. Sometimes rebooting the phone fixes it, but only for a while (and it's annoying to keep restarting the phone). Just wondering if there's another/easier way to fix this issue?

ALL I CAN SAY IS… DON't USE THAT IPHONE. USE ANOTHER SPARE CELL PHONE. AND SEE IF THAT WORKS.