Hey there 
This is similar to another issue which we handled in the week and might be due to the same reason.
In that case, the user was on an Apple M1 which has a WebGL2 maximum texture size of 8192.
This means that their browser was not able to load maps that are larger than 8192 pixels in either direction.
8192 x 8192 works, but anything larger in either direction would fail to load.
If you’re using a map that is larger than these dimensions, that might be what’s happening here. You could try to decrease the resolution of your map to fall within those bounds, reupload it (you can specify a different name so you keep both assets) and configure the scene to use the lower res version, and then it should load for that player.
You can ask your player to run WebGL Report and check the Max Texture Size field on the right hand side to confirm whether that is the problem.
Make sure that their hardware acceleration is turned on, since that might help to increase the max texture size their browser can display (though unfortunately it did not for the Apple M1 user I mentioned before).
Hope this helps!