OSX: Dupe-checking / Merging / Cleaning iCloud Contacts & importing 350 additional ones
A pickle indeed, data sanitizing is no fun and you have a doozy.

If you have access to his iCloud account you can go into contacts, select all of them and click on the gear icon to export them. I do believe that Apple only lets you export as a vCard.

I do believe there are a number of utilities that convert vcards to another format, some of them online.

Once converted I would be tempted to suck them all into an Excel (or other like Numbers, etc.) spreadsheet and do you data massaging there. But that would depend on your Spreadsheet-Fu. But getting it into a CSV format would be the first start. After that it's a matter of mapping all the right fields in an organized way so you can sort and find dupes.

Then it is a matter of getting the Excel data back into a CSV and then back to vcard format. Pssibly one of the online vcard to CSV sites do the reverse as well.

Hope that at least points you to a line of inquiry that works.