Why not just log into thier accounts and read thier private e-mails? Seems like that would be easier and then you could also see what is being sent to them.
Beyound that I would look in the inc directory. I think the file would be class.phpmailier.php but not sure. In that file you well see that there is a bcc array that is set up. Looks like it takes a combination of the address and name. I have not idea if this is right or would work but you could try setting line 223 which is the initiantion of the bcc array. It may be you will have to look at the CreateHeader function that starts around line 777 if you scan down some in the function you will see where the Bcc is handled. If you search for $this->bcc in the file you will see where the bcc array is used and modified.