Hey Christian,
If you can get your client or any colleagues to flag the fake reviews, too, that could help the process along. And if you haven't already, be sure to respond to the reviews. Even if you think they're fake this will provide damage control and hopefully limit the negative effect they have on potential customers. Just remember to respond politely, even if you believe the reviews are fake.
If the reviews haven't been removed after 72 hours, you can contact GMB support (via Twitter or Facebook has been most responsive in my experience). Make sure to provide as much clear evidence as you can that the review is fake and the reviewer hasn't visited your client's business. If you can, also explain how the reviews violate Google's policies.
Unfortunately, flagging fake reviews can be hit or miss, and if Google doesn't believe they have enough evidence, the reviews will likely stay intact. In that case, you'll just have to work extra hard to bring in some good reviews to dilute the negative impact!
Best of luck with it all
