Sharing secondary calendars from iCal to Outlook after sharing them to iCal

My colleague has 3 calendars on iCal under one email address. A while ago he shared them with me for my Apple Calendar app. But now I use Outlook for Mac as my calendar, as we also have an office 365 shared calendar we cannot see in Apple. I can see his primary calendar by looking for shared calendars in Outlook, but I can't see the secondary ones. If he tries to share his secondary calendars again by clicking on them, he can only see that he has already shared the calendar with me, and he can't get a link to share it again so that I can subscribe in my Outlook calendar.

All the online suggestions for sharing calendars from iCal to Outlook are for sharing for the first time, not for resharing. How can I get the URL again to subscribe in Outlook?