This argument sounds quite bizarre to me. Why would a visually impaired programer not manage to set their editor up to show indenting spaces the way they want, or failing that, collaborate with others to do so?

It's not like they can be effective at their job either without the ability to read 3rd party code, which overwhelmingly will be space indented.

How to set an editor to use big visual indents for spaces?

Depends on the editor, of course, but e.g.

https://www.emacswiki.org/emacs/redshift-indent.el

(Haven't tried the above, but I've done fairly extensive display hacks with emacs in the distant past, so I have a fair amount of confidence that it's not hard).