When you add the Google account it will create its own, separate, calendar in the Calendar app. You can see all calendars in the app by tapping on the Calendars button at the top left. Only calendars you choose (check marked) will be visible.
A newly synced calendar account will only overwrite your previous calendars if two conditions are met. One, this is the fist calendar you've decided to sync. Two, you choose delete instead of merge when the iPad asks you what to do with previous calendar events. Always choose merge when in doubt. It is easier to delete duplicates than create all new events.
You can choose the Google calendar as the default in the Mail, Contacts, Calendars settings. It will not delete or other wise affect the iCal calendar. The only thing that will do that is for you to turn off those sync settings.
From the way you are talking, I'm guessing your wife is syncing from iCal on the computer to the iPad, not using MobileMe. If so, she can turn off the syncing in iTunes if she wants to without affecting the iPad. The contacts will be erased from the iPad only if she goes to the Advanced box at the bottom of the Info sync page and chooses that option.
When creating a new event, you can choose the calendar in the edit dialog. You can also do this after an event is created, if you just want to change it's calendar.