User question: Can I share a calendar with someone who has no next/owncloud-account?

Perhaps a stupid question:
Is it possible to share a (nextcloud) calendar with someone who has no next/owncloud-account?

In my simplicity I have so far assumed this – but when I send a calendar link to my friend (Mac user with no nextcloud account) he runs against locked doors.

Is that just so - or am I doing something wrong?

Thanks by advance
Frank

What does, “runs against locked doors,” mean, technically-speaking?