Unfortunately I'm using SA with sendmail + spamass-milter. I can't use common procmail filter because I have to forward messages to M$ Exchange and store copy of message locally. It's easy for me use sendmail aliases than ~/.procmailrc files for each users. Only one person don't want use SA. Of cause he is M$ sysadmin.
With plain spamass-milter, I'm pretty sure you're not going to be able to fix the problem, but I'll admitt I'm not all that familar with spamass-milter.
Since you need to stay at the MTA layer you might want to consider MimeDefang, which one user suggested can be forced into splitting multi-recipient messages up. From there you'd want to configure MimeDefang to not call SA at all for your M$ friend. I'm not very familiar with MimeDefang, but I'm pretty sure this is possible as I know the tool is VERY flexible.
Another option is MailScanner. I know several people use a sendmail trick to force it to split the messages before mailscanner gets them. MailScanner's config can then be told to not spam scan for a given user, or any kind of simple wildcard based rule on the envelope recipient. MailScanner is also MTA layer, but operates by queuing mail via sendmail, scanning it as queue files, and then passing it back to sendmail which can relay or deliver.
