Hi Ira,
It's a very good question and it is currently a way that companies are trying to "Cheat the system". From what I have experienced in the past, the company in question are just making the best of Schema as it is meant to be used. For this reason they are theoretically not doing anything wrong in Google's eyes.
If there reviews are genuine then there is no reason why they can not be used and your client is within their right to use the same tactic.
If you are genuinely worried that this client is doing something wrong or unethical/fraudulent I would give Google a call directly and raise it with one of the technical team. I would mention that you have seen the site using this and provide them with the example, just explain that you are thinking of using the technique but you wanted to check first that you are not doing anything wrong.
The team at Google on phone support are great and they have always been able to help with any questions I have had in the past.
I hope that helps, good luck finding a solution.