I've never observed the phenomenon you're describing, perhaps because I've never needed that precision.
However, how about Find/Change for that Unicode glyph in the that particular font, and replacing it with a thin space in a font that doesn't have that Unicode glyph.
You could set a character style with only a horizontal scale value set to 12.5%. Then apply (with a grep style if you wish) this character style to any em space that you need to be only 1/8th of an em space.
Thank you for your answer. This is indeed a possible solution, but I don't have the time to do that in my situation (I'm working for a newspaper, and I'm not the only one working on InDesign).
The solution from Skemicle is also time consuming (first I would have to insert an em-space for each occurrence).
If it were up to me, I would change the width of the glyph in the font, with a font editor, or I would delete it. I'm just disappointed that there is no simple solution (like a checkbox). QuarkXPress, for instance, does not take account of the thin space in the font.
The thin space is a fifth of an em. So for a 12pt em, for example, it would be 2.4pt wide.
In English, yes... (although this page is saying : "One‑eighth the width of an em space")
But not in French :
"Thin space U+2009 - Standard setting is 1/5 of the em space. 410 units in a 2048 unit per em font. This should be language dependent. The standard language dependent setting for French is 1/8 of the em space. 256 units in a 2048 unit per em font.
Note : When traditionally typesetting the French language a word space is inserted before or after several punctuation characters. These characters are colon, semi colon, question, exclamation, right guillemets, and left guillemets. Commonly the preferred word space used is a thin space of 1/8 the em."
So when I'm using a font made by an English speaker, I often end up with a thin space of 1/5 of an em, which is too wide...
Seems to me that since the 1/8 em space is required with specific characters it would be fairly easy to create a GREP style to apply a character style that gives the thin space a horizontal scale of 62.5%. This is basically the same as skemicle's suggestion, but using the thin space rather than an em space.