Hello @Piotr,
There are a couple of suggested third-party viewers in this thread, like Speckle, and one that is considered the “semi-official” solution by Rhino called Rview. The effort seems to have been targeted at IRhino, the official iOS viewer by Mcneel, which looks very well featured but sadly excludes Windows/Android users.
Alternatively, I had success exporting to a common file format like STEP and opening those files in any free viewer by the “big boy shop,” AKA Autodesk, SolidWork, etc.
Does your client prefers a web application or a free, downloadable desktop application? The desktop application could be from the same author as their current CAD tools that they are familiar with.