How to sync yahoo calendar from BB Passport for two-way editing on other devices?

I've recently become responsible for managing my boss' calendar. He uses a BB Passport device that is synced to yahoo mail. I want to use both an android phone (samsung s7) and pc computer to manage his calendar. I have gmail and outlook currently (the s7 does not support a yahoo calendar).

I shared his yahoo calendar with my outlook but after the initial connection, it is not continuing to sync, nor is it two-way (though I did shared it so I could edit). Events I create on his web-based yahoo calendar are not syncing to either my outlook or to his BB device. And events I create on outlook are not syncing back to his BB device or yahoo calendar.

What is the best way I can get two-way access to his BB calendar? I've seen mention of CalDAV and Outlook but don't know how to proceed. Is there any scenario in which he can continue to use his yahoo calendar on his BB but i can also have editing access? Or at minimum, use yahoo email that links to a calendar I can share (such as google/outlook/etc)?

I've searched every forum I can find but am pretty stumped on how to proceed.

From my experience, Yahoo calendar and BB10 have never worked well together. He doesn't have an Outlook account he could use?