It's true that the space between sentences should be larger than the space between words, as it is in properly typeset text, though not twice as large. Using two spaces is a way to approximate that when you only have monospaced fonts. It's no longer encouraged in electronic text when using proportional fonts because the rendering software _should_ adjust the spacing itself.
It "should" adjust, but in practice most software do not render spaces after periods properly. There is no in band signal to distinguish periods for sentence ends vs e.g. Mr. Thus there is a clear argument for double spacing in all contexts
Buddy, there are rules and there are recommendations. The fact that it was common practice to use two spaces after a period up until the early 2000s in many situations shows that it is not in fact a rule, which is linguistic (as in the capitalization rules for proper nouns), but a recommendation, which is stylistic.
Stylistic recommendations can also be taken as rules, but in limited situations, such as a place of work saying we follow these guidelines (a manual of recommendations), which then become rules for that workplace.
Finally this link https://en.wikipedia.org/wiki/Sentence_spacing shows that their complaint has some merit, because the spacing used are not rules of the language, but conventions or recommendations imposed to improve the production of text based on the requirements and limitations of a particular technology.
on edit: probably shouldn't describe capitalization as being a linguistic rule, as it is an orthographic rule...heat of the moment, sorry. One might argue that the same applies to spacing, but I suppose that capitalization in orthography has been established for so long, and in so many different systems that it can be claimed a rule whereas spacing is still just a recommendation.
