I might not meet the stated qualifications for replying... and I am not going to talk about algos... but I will say that I have very carefully watched a small number of sites for over a decade and have experienced several huge losses in income caused by something that Google had done. So, from the standpoint of wallet smoking, I feel that I have a good reply. I have also been a student of Google - and shareholders.
What I am going to explain here is a long-term trend for Google. Google used to be a very crappy webmaster. They gave all of their traffic away. Google used to be a revolving door. Traffic came in and went out. Google kept little of it. Google monitized only a fraction of the searchers.
Over the past few years google has become a better webmaster and has become better at keeping traffic on google.com or giving a higher percentage of that traffic to people who are paying them.
Shareholders want revenue growth. Google has owned almost all of the USA search volume for over a decade. So they are not going to get growth from claiming more marketshare. Instead they are going to get growth from being a better webmaster. They are going to get growth out of making more paid clicks. That means your clients and their organic or local traffic are in the crosshairs. For the past several years they have been the targets of google's revenue growth. That is not going to change. Your client's current profits are google's future growth.
OK... Where has made my wallet smoke? One day I wake up in the morning and find one of these. These traffic losses are not temporary, they are not recoverable, they are permanent. BAM!
When my income has taken the greatest hits.. the hit was usually NOT caused by an algo, it was caused by a change in the format of the SERPs... and since my competition is usually confined to organic listings I have been clobbered by these..
-- In the early days of Adwords, the ads were only displayed on the right side of the SERPs. When Google got the smarts to display them above the SERPs my retail income was hammered.
-- One day google decided to start displaying a sample of the image search results near the top of the SERPs. I have a couple sites that are heavily based upon images and those sites lost an enormous amount of traffic. Tens of thousands of visitors per day disappeared overnight. More wallet smoke.
-- Then google added "Product Search Results" to the top of the SERPs. My retail sales for pages ranking in the top three postions of the organic got hammered. I decided to jump in to product search. Did that and got some traffic and sales back. Now product search is pay-to-play. Google grabs some of my profits if I want to play or some of my total revenue if I ain't going to play.
-- Then google decided to change the image SERPs. Instead of the visitor clicking on my image and being delivered to my site they started to show my site in a frame. Lost a little income that day. They they decided to display my image on a page of google.com and only have a tiny link to my site.
-- One day I wake up and google is showing a big map at the top of the SERPs. Sometimes it has pushpins, sometimes it doesn't. Familiar smell of currency and leather smoldering in my back pocket.
-- One day google started showing carousel at the top of the SERPs. Now, instead of the visitor clicking straight to my site it clicks straight to more SERPs.
-- This year Google started adding knowledge box to the top of the SERPs. (Just my weak minded observation... when they did this they demoted a large number of informational pages. Go search for "jade" the green gem. Not long ago that SERP was filled with oriental history pages, museum pages and pages of vendors selling the green gem. Now the organic SERPs on the left side of the page are very different. Wikipedia is there but the museums, sellers and oriential culture pages are gone. Instead they have a knowledge box that answers light queries, If you are lucky your site might appear in the knowledge box. But they have obvious links to other Google queries in large BLUE font the knowledge box but links in microscopic black font to your website.
OK... Now you ask about what to do for your client. First. Explain that he has a little algo risk... and that he has a little competitor risk... but also explain that even if you achieve the miracle of getting his website to one of the top ten positions of google that he will still be subject to a much greater risk for which you have absolutely no control. That is SERP format risk.
So, I would take a screenshot of his target SERPs on the day that he signs your contract... and when he calls you about his wallet smoking you will be able to show him that google has changed the playing field by changing the format of the SERPs. Your clients profits are the field that google is plowing to get the growth that their shareholders demand.