Linda's right, unfortunately. Google is not going to look too deeply into this because the content of the reviews is not in violation, and they are not all coming from the same account.
Personally, I think this person is creating Google accounts, and reviewing under them. Google requires a phone verification to create accounts, but with enough friends and family to use their phones, you can do this indefinitely.
Looking at all of the accounts, every one of them is either a first time reviewer, or made multiple reviews in the area very close together. All of this together tells me that this is a single person, who actually may have knowledge of Google's TOS.
I see that the business founder replied to the latest one... I would advise deleting that particular response, and making a new one that is a bit less defensive. Something that starts off along the lines of "I'm sorry, we would like to reach out and try to rectify this situation, but we do not have your name in our system. I'm not able to locate where we have provided services to you. Could you be listed under a different name?"
You can also address the contents of the reviews with facts - calmly and professionally. "We do occasionally hire contractors to service our jobs during high volume times. If there is an issue with the job they did, we certainly need to hear about it. Unfortunately we are not finding you in our system, and so cannot see who worked on your home. Could you have possibly signed on with us under a different name? Please reach out to us directly at xxx-xxx-xxxx, and we will be more than happy to get this resolved for you"
Responding in this way individually to each review lets new customers who see the reviews see that you take responsibility for the work that you do, and are willing to make it right... it also lets them know that this may not be an actual client of yours. It will leave them with a better impression than a more defensive response would leave.
This, along with burying these bad reviews with good, is most likely your only option because Like Linda said, Google is most likely not going to remove them. It's very unfortunate, and I am so sorry this is happening to your client's business!
-Cherie