Where is the Contacts data file kept?

It's not hiding. It is located in /Home/Library/Calendars/ folder.

Four Ways to Make the Home/Library Folder Visible

A. This method will make the folder visible permanently. Open the Terminal application in the Utilities folder and paste the following red colored command line at the command prompt: chflags nohidden ~/Library then press RETURN.

B. Click on the Desktop, press the Option (⌥) key, select Library from the Finder's Go menu.

C. Select Go To Folder from the Finder's Go menu. Paste the following in the path field: ~/Library and click on the Go button.

D. In Sierra activate the Finder, press COMMAND-J, set Finder display to Icon view if it is in some other view mode, select your Home folder from the sidebar, check the box labeled, "Show Library Folder.

Is he using iCloud for his contacts?

Are his Contacts present if you log in to iCloud on its website: https://www.icloud.com/#contacts?

For Contacts go to your Home Library:

Home/Library/Application Support/AddressBook/Sources

Take a look at this User Tip: Fix corrupted Contacts database

Durn! I don't know why I wrote "Calendars." Should have been "AddressBook."

DarleenMB wrote:

I wish that had solved the problem but it didn't. The metadata is all there but Contacts 10 won't read it.

Were the contacts in an iCloud account or On My Mac?

Maybe I should try reinstalling the previous version of Contacts.

No, that won't help.

If the metadata is there, then you can get it back showing in the index, but since you said my instructions didn't work, I need to know more about the account to help you fix it.

Yes, I did miss that, but reading back through I see where you alluded to it.

Can you open the AddressBook folder and take a screen shot of it (cmd-shift-4, then spacebar, and click on window).

Post that screenshot in a reply here. The file is in the Desktop folder. Just drag it into the reply window here.

Looking at your screen shot, delete everything above the Images folder.

The, open Contacts and see if the contacts reappear.