How about adding in the head's calendar the assistant in the list of notified persons when the calendar is modified? That should do it.

This would be a solution, if the secretary could see all events in that calendar.
But in our case this is not the case, as there are some private events in this calendar too, which the secretary shall not be able to see at all.
With the above solution the secretary would get an email for every change, even the "shall be invisible" private ones.

Therefore we need another solution.
In a private conversation Ludovic suggested:
"When generating a mail back to the organizer when the attendee
accepts/declines, we would also send a mail to the SENT-BY. I suggest it
being a purely "text" email - with no ics attachment to avoid confusion
of MUAs."

With this solution the secretary will only be informed on accept/decline of attendees, when he/she has created the event with invitation.
For us this is the desired solution.