A friend of mine asked for some advice on sharing a google calendar.
I wanted to help, but could not find a way to share it with an email address for someone he’d never sent an email to.
I can add the email address in the sharing options, and select the access rights, but the “send” button will stay greyed out.
Is it a regular gmail account or work G-Suite account?
You should be able to do it. Instructions here
The recipient must have a Google Calendar account; if not the calendar must be made public.
Maybe a little off-topic, but Woven might be an interesting app in this regard. It is a calendar with complex scheduling options through links in email or even iMessage.