I have a gmail account as well, and I know that it doesn't handle mailing lists well. However, gmail does offer POP3 access you one's account, so he could always set up mutt or icedove or any other MUA that can handle POP accounts, then he can reply to the list properly.
I also use gmail for lists, and you can avoid problem using 'Reply all' and erasing user's mail address leaving only the list. have a try cheers raffaele
