Photographer writes edits to XMP (Metadata->Write Metadata to Files) and sends the RAW photo and associated xmp file to you. You do the same but you only have to send the xmp file back. Photographer replaces the original xmp file with the one you sent, and then Metadata->Read metadata from Files.
Fantastic - I knew there would be an easy way! Cheers.
Another option to consider - have them export a catalog and send to you. When finished, you send the catalog back and they import it. Either way accomplishes the same goal but catalog export / import obviates the need to fiddle with xmp files, and if your "client" uses DNG, obviates the need to send big photo files back, unless of course you Photoshop'd the raws and thus spawned tiffs or jpegs for them, in which case you have to send them back no matter which way you go..