With the best of luck your client might be using Google or Cloudflare’s DNS servers, and after a recent (a few hours or minutes) DNS change they have stale information.
Have no fear because by the luck of Zeus and Edward (Eddie) you can use these URL to flush the cache:
https://developers.google.com/speed/public-dns/cache https://1.1.1.1/purge-cache
For those interested, we are referring to these DNS servers:
8.8.8.8 (Google) 8.8.4.4 (Google) 1.1.1.1 (Cloudflare) 1.0.0.1 (Cloudflare)