Hi Robert,
Excellent answer- Many thanks for offering your input.
Unfortunately, I don't have access to the Analytics accounts of other sites my client is referring to, so it's difficult to validate that statement he made. As this review content isn't actually placed on the actual pages of my clients site, is Google capable of acknowledging it as duplicate content? Is it even crawlable within its current form?
It doesn't appear that my client has been doing any form of link building with the site having relatively few linking root domains. Also in terms of on-page copy, they have been adding unique copy per product on the site.
The aggregated reviews could potentially be the main issue but it does seem to be a very quick drop in traffic if they had only been added a couple of weeks ago. I do agree with you though that they should be getting their own consumer base to submit reviews, even if the volume of them won't be as comprehensive.