If a user Favorites a Tweet published by a user that you are subscribed to while that subscribed user is private, we WILL deliver that activity to you via the Account Activity API.
If your subscribed user is private and Favorites another Tweet, we WILL NOT deliver that activity to you via the Account Activity API.
If your subscribed user is NOT private, any Favorites of their Tweets or their Favorites of other Tweets will deliver via the Account Activity API.