Kerning issues with GREP Styles?

Are you both using the same version of the font?

Have you done the usual troubleshooting steps: Export to .idml, open and save as a new .indd and trash the prefs on the offending machine: Reset InDesign Preferences and Other Troubleshooting 

I think I might repost this without the GREP connection as it's nothing to do with GREPs

Update: The sub and I have gone back to earliest versions. In my layout the kerning is fine. When she receives it and opens it there is a -200 kern applied to space character before the hyphen (actually en-dash tbh). She is on the latest (and same) version of indd and she swears it just opens like that without her doing anything to it or the file.
Two grabs attached: the first one is grabbed by me just before sending it, the second is grabbed by her just after simply opening the file.

And how about your justification settings? Could be that the elimination of runts is forcing the tightening of spaces in generalin some paragraphs and you only notice it around the hyphens.

It does seem quite unlikely that the GREP style is causing it, doesn't it? But you could always test it by turning the GREP style off (perhaps by duplicating the style, editing the duped style to have no GREP styles, then applying the duped style to a paragraph with the undesired spacing behavior) to prove to yourself whether or not it was the regex that was causing the issue. Likewise, you could export to IDML and re-open and re-save to see if was some kind of file corruption that could be resolved by roundtripping through IDML. 

I'd also observe that a new version of InDesign was released recently; you said that "literally nothing else has changed" but you didn't tell us what version you folks were using. 

Lastly, I think you'd get more useful responses if you posted both a screenshot of the text with the poor spacing (preferably with both View -> Screen Mode set to Normal and Type -> Show Hidden Characters turned on), as well as a sample page in INDD so that we can download it ourselves to see if this problem is something related to your fonts, or to your install of InDesign.