We have taken steps as drastic as changing their email address and creating a completely new account.
When you setup a Google account (and enable Calendar by default), your Gmail calendars should sync without a problem.
Apple does not allow “push” notifications from Gmail (or any IMAP-based email) app.
A “Push” notification is where the app automatically detects when there's a new email and pushes a notification about that.
The issue is not even reported with latest Apple flagship i Phone X with i OS11.
Even on the latest i OS 11.2.5 yahoo emails are not updating.
These days, the setup is so easy that it all happens automatically once you sign in with your Gmail/Google credentials.
But there's this remote possibility that Google/Gmail calendars are not synced to your i Phone's calendar and that can be a big problem.
There are two other options available for checking for mail: fetch and manual.
Fetch is where the app checks for new email frequently.
In case you can’t wait till Apple fix for the issue, you can Shifting to using Yahooo Mail App.