Opening Smart Preview files with the lrdata file

I assume you Imported the DNG Smart Previews into your own catalog. As a result your LrC sees the images as new independent DNG files that bear no relationship to the Smart Previews provided by your client.

This can work, but - *The Client's Smart Previews.lrdata file must NOT be modified in any way!

1) In your catalog EXPORT 'DNG' files with the edits to another folder also with a Rename, and use these 'exported' files in the steps following.

2) Close your Catalog

3) Open the Clients Catalog. It will require the original Lightroom Smart Previews.lrdata file exactly as the Client provided to you.

4) Import the DNG files that you had edited and Exported in Step#1.

Now you should see two versions of each image- one being the Clients Smart Preview and the other being your edited DNG files.

5) Copy and Paste Develop edits from the edited DNGs to the Clients Photos.

6) Now 'Remove' your DNG files from the Clients Catalog.

7) Return the Clients Catalog that now contains edits copied to their files. They will {Import from another Catalog} to merge the editing into their master Catalog.

This is what I see when I Imported some edited DNGs to a 'Client' catalog-

The Client Smart Preview image + my edited DNG image:

After doing a Copy&Paste of Develop settings DNG > Client image.

There is sure to be a 'Syncomatic' Plugin available that can probably do an easier sync of edits!