In the book she exhales:
"She pushed her jaw forward in a yawn, opening her throat and her Eustachian tubes. Cyn yelped as she hit OPEN OUTER DOOR. Air tugged at her once, hard, as it evacuated. Adrenaline flooded her blood as she was assaulted invisibly on every square centimeter of flesh. The breath in her lungs rushed out of her, trying to pull her lungs along with it.[...] With her lungs empty, there was no reserve. She wasn’t holding her breath, surviving off the gas held inside her. Someone could hold their breath for a couple minutes. In the vacuum, she could make it maybe fifteen seconds unaided.
Corey, James S.A.. Nemesis Games (The Expanse) (p. 425). Orbit. Kindle Edition.
If you're referring to the scene where she deliberately exits an airlock, then she actually exhaled whereas the other person with her did not and died.
(Purposely being vague to avoiding spoiling anyone on this great series)
I rewatched this moment on Prime and I remembered it correctly. While she exhales after last words slowly, she inhales sharply before pressing the button.
I remember it probably because I watch everything with subtitles on and that was written. I now watched it without to avoid being biased and it still holds.
In addition she was exposed to the vacuum about 40 seconds before using the shot.
Corey, James S.A.. Nemesis Games (The Expanse) (p. 425). Orbit. Kindle Edition.