Customer migrating to microsoft exchange for email

Yup, you'll need to update the MX records for their domain to point to their new mail server. You'll probably also want to update anything else that might be pointing at Google Apps (e.g. mail., smtp.). This should be done in cooperation with the new e-mail administrator to minimize disruption. The change shouldn't affect anything else.

also, if they are planning on utilizing the autodiscover service, you'll probably need to add an A record for autodiscover.clientsdomain.com. The new email guy should have all of this info for you though.