Not sure how your user's iPhone is set up but:
It’s not one without a solution however, with GO Contact Sync Mod, you can keep your Outlook and Gmail contacts in check. If you make a change to one then you can use GO Contact Sync Mod to update the other, quickly, conveniently, and almost effortlessly. Why Outlook over Gmail? Being a Google mail client it is a more secure platform. As Google does not have webmail, it becomes easy to sync MS Outlook with its mailbox items. It allows you to send/receive emails, schedule meetings, storing contacts, etc. It does offer many more features than you use daily. Things to remember while Syncing: 1. Pro Tip: Go to Settings Google Account services Google Contacts sync. Check for the status under Sync status. If it’s showing off, tap on Manage settings. Enable sync on the next screen. We would like to show you a description here but the site won’t allow us.
1- If Outlook for iPhone is installed and syncs properly, it will not propagate to the native apps like mail, calendar and contacts from the iPhone
2- The account also has to be configurerd on the iPhone with only the items that they want to sync selected, though I fail to see the usefulness of
synchronizing the calendar twice, I usually only sync the contacts so I can have the phone numbers available on the iPhones (or Androids)
Gmail Contacts Not Showing On Iphone
3- Check the sync period, it may take a long time to sync if the period is too long, usually a max of 30 days is more than enough to stay on top of things
Gmail Contacts Not Showing In Windows 10 Mail
I think it's also worth mentioning that calendar sync's have always been troublesome between Office 365 and iPhones.