The review system allows reviews to be published automatically and immediately, so if someone says that they submitted a review, then they either didn't click on the "submit" button or they might not have given it time to actually submit before leaving the "submit a review" page.
What we suggest is:
1.) Refresh yourto make 100% sure the review isn't there, and
2.) If it's still not there, ask your client to try again and also suggest that they should maybe try from a laptop or desktop PC (if they are using a mobile - some devices are not compatible with the latest technology). This is beyond our control 🙂