iPhone doesn't sync calendar to iCloud, calendar via iCloud.com still works
Try creating a new calendar within her iCloud account. Don't share it yet. Add, edit, and delete events both on her iPhone and iCloud.com through a browser. If this works as expected, share the new calendar with your iCloud account. Repeat the add/edit/delete cycle to see if changes propagate across all devices now. If this works too, use the new calendar instead of the old calendar going forward.