How can I get Siri to pronounce Exchange contact names correctly?

You can create iCloud contacts that merge with your GMail contacts. The iPhone will collate the data from both the GMail contact and the iCloud contact. I have this enabled for Hotmail:

1. Enabled iCloud syncing for contacts.
2. Open Contacts.
3. Press the back button (top-left) until you reach Groups.
4. Open All iCloud or All from My PC.
5. Create a new contact for your wife. Include her full name and phonetic name.
6. Press Done.
7. Scroll down and check that Linked Cards shows an entry for Unified and GMail. Press Unified.
8. Now you can see how your wife's information combining the GMail information and the iCloud phonetic information.