@jonathanleplang Hi Jonathan, sorry to hear about your client's predicament. Review loss of this kind typically happens for two reasons.
One, there's a bug in Google. Check the Google Business Profile help community and Sterling Sky's Local Search Forum when review loss happens to see if others are reporting a similar experience. There have been historic periods in which Google has accidentally made reviews disappear and then reinstated them.
The second cause is that Google has removed the reviews because they became suspicious about the profile, for some reason. For example, here's a recent article about a business losing the majority of its reviews because of review gating: https://www.sterlingsky.ca/google-removes-hundreds-of-reviews/ and another about why new reviews might not be publishing: https://www.sterlingsky.ca/google-reviews-not-publishing/. Read those with the client and try to root out whether they might unknowingly be violating Google's guidelines (see: https://support.google.com/local-guides/answer/7400114?hl=en#zippy=%2Cadvertising-solicitation%2Cfake-engagement)
If you are unable to discover the cause of what is happening, it's time to contact Google about the case. Good luck!