Double spaces after a
sentence was the
standard for using
typewriters before the
computer was commonplace.
If you look at old typing texts, they said to add the doubled spaces.
In actuality, it depends on the font style used.
Some
fonts were not monospaced.
A monospaced font is one that has every letter taking up a specific
equal amount of space.
With the advent of computers, monospaced fonts have been replaced by letters that do not take up the same space.
For example:
Pack my box with five dozen liquor jugs.
Pack my box with five dozen liquor jugs.
Note that the upper sentence (made up of every letter in the alphabet) takes up less space than the lower sentence.
This is because the upper font has letters with different spacing and the lower font is monospaced.
Therefore, you are both correct.
In typing on a typewriter, use double spaces after sentences.
On a computer, use a single.
I'm a tech writer, and all of the contracted manuals I wrote required the new computer standard (single-spaces between sentences).
Hope this helped.