He can save as IDML.
CS6 is part of Creative Cloud. Have your co-worker use that.
If you are also going to be working on just a single file and not round tripping back to the designer, have the designer do the idml export.
But if there will be collaboration or more files, have the designer install and use cs6 for your work. They can install cs6 at no cost as it is available with their subscription. Then share the actual ID file.