Well, that's your biggest problem then. Relying on organic SE traffic for every bit of traffic to your client's sites is a highly risky -- one is tempted to say reckless -- strategy.
Search engine algorithms change all the time. Google puts through MORE than one algo update every single day. You simply cannot predict when one (or more) of those changes is going to have an adverse effect on the client's site.
It may not be the result of anything they've done "wrong." There have been many instances we have heard of here where legitimate business sites get caught in the crossfire. They become what is sometimes referred to as "collateral damage." Google didn't intend to slap down that particular site, but because it's profile was similar to sites they were targeting, the legitimate business site is dropped down or removed from the SERPs.
When that happens, the business owners flock to forums such as this, wailing that unless they get their search traffic back they're going to lose their business. Employees will be laid off, suppliers will go unpaid, the business owners will go bankrupt.
And we have to tell them, there's nothing we can do. They can file reinclusion requests and complain on forums and write angry letters to Sergey and Larry, but in the final analysis there's nothing anyone can do to make Google list their pages. Google is under no obligation to list anyone's page at any particular position. If they choose to downgrade or eliminate pages that meet certain criteria, that's their prerogative, and nobody can force them to do otherwise.
(In fact, more than one company has taken them to court to try to do just that. Every company that has tried this has failed. The SERPs are Google's property and they can list -- or not list -- whoever they want.)
All we can do at that point is tell them they need to investigate other marketing channels. However, when you're facing business collapse, that's not the best time to try to jump into new marketing initiatives. Business owners need to have set up accounts, made contacts and developed expertise in these other channels before they really, really need them.
This client must implement other marketing tactics beyond SEO if they want to remain viable as a business. You simply cannot rely on organic search as the only source of traffic for the site.