As long as the MS SBS server will readily accept an incoming connection at any time, then yes, that's what he's referring to. Your server would just automatically forward email for that domain to the SBS server instead of putting it into local Maildir's.
/var/qmail/control/smtproutes file would contain lines such as:
domainname.com:11.22.33.44:25
where 11.22.33.44 is the IP address of the SBS server, and 25 is the SMTP port (in case they are not running on standard port 25)
And you will have to remove the domain name from the /var/qmail/control/virtualhosts file or Qmail will try and store the domain's email on the local Plesk server.
And the domain name should be in the /var/qmail/control/rcpthosts file, or Qmail will reject that domain's emails.