Hi, Dan
There isn't currently a way to prevent the manager from getting the iCal invite. The only way the Manager would not receive the iCal invite is if the Administrator who's booking the user onto the Face-to-face session, or the user themselves, choose to only be sent the email notification without the iCal attachment.
There is a way to disable the iCal notifications for cancellations, but that would be disabled for both the user booked onto the session and the manager receiving the notification.
Would I be right in thinking that the iCal is being added to the Manager's calendar as well, hence the request? If so, I think this is a good point and I can certainly file an enhancement for this.
Cheers
Tom