Outlook appointments don't sync to iphone if recipient accepts on iphone instead of computer?

We are having an issue where someone here will initiate an appointment in Outlook. He will invite someone as an attendee. If that recipient accepts it in Outlook, then there is no problem. It will show up on the senders Outlook and iphone. But if the recipient accepts on her iphone, then the appointment only shows up on the sender's Outlook calendar, not his iphone.

Any ideas?