For your /e/ phone, we will need an additional app … Anyway this will not give two-way write capabilities 
Go to the Account Manager, then to the settings for your /e/ account :
Then click on your account, then Webcal :
Swipe down as requested, the Google calendar should appear :
Note : the calendar list is being fetched from /e/ cloud, but you can also add some manually
If you tick the case a left, you will be asked to install ICSX5 :
Please don’t click the link (it won’t work), instead please point your browser to F-Droid and download it :
As the browser is not allowed to install packages, launch Files app and go to Downloads or search for the downloaded package :
Then install it, launch it, wait for repositories to sync, search for ICSx, install it :
No need to launch it at this time.
Go back to Account Manager, tick the case again, allow access to ICSx5 :
Click on the arrow at upper right :
Change the color and the display name as you wish, validate at upper right :
Back to the Account Manager, you should see a “Subscribed successfully” for 1-2 seconds, then refresh using the button et bottom right :
If you open the ICSx5 app, you can choose sync frequency :
You /e/ phone Calendar should now display your husband’s Google Calendar events :
