Thank you for using Apple Support Communities. It's my understanding that your grandmother isn't receiving Messages notifications on her iPhone. I'm happy to help.
First, check the sound settings in Settings > Sounds. If the issue persists, use the steps in the section "If you don't see notifications for a specific app" in the resource below:
Use notifications on your iPhone, iPad, and iPod touch
If behavior continues, restart your iPhone. Restarting can resolve many unexpected behaviors.
Restart your iPhone, iPad, or iPod touch
Have a wonderful day!