Your friends can see EVERYTHING you do even when sharing is disabled!
Log in to your YouTube account. Go to the main page http://www.youtube.com/
You can go to the main page by clicking on the YouTube logo on the upper-left side of your channel.
On the main page there is a "Recent Activity " box where you can see all your friends's activities.
But, your friends can also see all YOUR activities even when sharing is disabled!
To see your sharing settings click "Account", then "Activity sharing".
YouTube is already aware of this problem because I posted a question on Google Help Forum here:
Let's complain and ask YouTube to fix this!