If you have Send= anything but Public, he's not going to be able to post
from the non-subscribed address.  LISTSERV does not do "domain-munging".
 
Possible solution:  He <could> set his POP client up so that the return
address on his mail is [log in to unmask] (or whatever his subscription
is for), OR you could add the offending address to the list and set it
to NOMAIL.  I prefer the former to the latter.
 
Nathan