I think this guy needs to get clear on the distinctions between typewriting, typesetting, and word processing. If you're typing on a typewriter, it sounds like it's generally considered admissible to use two (monospaced) spaces after a word.
As far as typesetting goes, TeX makes an inter-sentence space a bit wider (but less than 2x) than an inter-word space, unless you tell it otherwise. Since I trust Knuth more than this guy, I suspect that's at least considered typographically admissible.
If you're using a word processor with a monospaced font, it seems like it makes sense to follow the typewriter convention (two spaces between sentences). If you're using a word processor with a proportional font, and you want to follow the TeX typesetting convention, I think you're stuck: it's not trivial for a computer to distinguish between an inter-sentence space and an inter-word space (because of things like "etc."). So if you use a single space between sentences, it's going to be smaller than it would be in the typeset document, and if you use a double space, it's going to be larger than it would be in the typeset document. Or you could conveniently decide to follow the "french spacing" convention, in which inter-sentence spaces are the same size as inter-word spaces. Which is apparently what the author favors.
As far as typesetting goes, TeX makes an inter-sentence space a bit wider (but less than 2x) than an inter-word space, unless you tell it otherwise. Since I trust Knuth more than this guy, I suspect that's at least considered typographically admissible.
If you're using a word processor with a monospaced font, it seems like it makes sense to follow the typewriter convention (two spaces between sentences). If you're using a word processor with a proportional font, and you want to follow the TeX typesetting convention, I think you're stuck: it's not trivial for a computer to distinguish between an inter-sentence space and an inter-word space (because of things like "etc."). So if you use a single space between sentences, it's going to be smaller than it would be in the typeset document, and if you use a double space, it's going to be larger than it would be in the typeset document. Or you could conveniently decide to follow the "french spacing" convention, in which inter-sentence spaces are the same size as inter-word spaces. Which is apparently what the author favors.