If the information you are referencing has been obtained from the 6th edition, then you ought to reference that copy rather than the older one.
If you are following the Cite Them Right referencing style, then you would reference the chapter author as outlined on the subsection “Chapters or sections of edited books” (under Books - Collections and parts of books).
Following the instructions, Mike Fray should be listed as the author, if you are referencing the 6th edition.
