Google. What would you recommend the website marketer do in response?

Check to see if Google has announced an algorithm update, which they sometimes will do$.$

Check the Mozcast turbulence report to see if there are fluctuations in listings around the same time as he noticed the decline of his own pages' listings.

All of these are recommended responses.

Assess the website's backlink profile to determine if low$-$quality or spammy sites are linking to it$.$

Check SEO authority expert sites like Search Engine Journal to find out if they suspect an algorithm update has occurred.