Actually not the email would not be going through his servers, it's directed through Google's When I set u my account, I had to have Hostgator manual make the changes for me because they don't have the ability to change the Mail Servers available as a default.
Your client will typically see a blip in receiving his email for any up to 24 hours as the email servers are updated but that's it. Then he'd have to set up his POP3 accounts again to access his email via Google. (if that makes sense, I'm writing this with a Head Cold Fog
So with the "Free" Google Apps you client could have multiple email accounts and each of you would have access to the docs, calendars and such he just wouldn't be able to access the marketplace but for many that doesn't matter...the comparison between the free and business app (Paid) is shown here - http://www.google.com/apps/intl/en/business/smbs.html
, it's still showing 50 emails.
Hope that helps some...