How can I sync Apple Calendar with non-Apple devices?

Hello.

How connect my calendar with my assistant if she hasn't got apple devices, only Windows фтв Android.

Apple Calendar synchronize with my Google Calendar and after synchronize my Google Calendar with her - doesn't work and too difficult.

I've registered her in iCloud with out apple devices. There are Notes and etc., but there is now calendar.

I don't want to use Google Calendar. How to synchronize Apple Calendar more simple?