One of my coworkers (F27) (I'll call her Sina) is quite shy or at least quiet and never complains, but she does her work well. Unfortunately another coworker (Monika) (F34) took advantage of that and often gives her part of his workload to do as well (and tells her to give it back to her to commit), which causes Sina to fall behind with her own work. Also, Sina does her work better, so Monika benefits from that as well, and when Monika does a shit job, she'll drop the task on Sina last minute and then lets her commit the faulty task. 

Now, I'd usually try to stay out of this (or at least wait until I have a good way to prove it to the boss, since I can't just record it in office), but here is the thing: From the looks of it, Monika is bound to get promoted once my boss retires (partly because of the work that Sina did for her). Unfortunately for me (or for her in the end), she's a very bigoted person that always talks shit about men in the company workforce, saying we are lazy and do a shit job, and that there should be more women anyways (I don't disagree with the last part though). At the same time she is the one who steals someone elses work and time... If she were to become boss, it could be quite problematic since I'm trying to get promoted to a different position where I need the votes of at least 3 company bosses (position my current boss holds). And of course I don't want a stupid lazy bitch to become my boss. Meanwhile I'm on good terms with Sina and I expect her to be more professional in said position.

When Sina went on holiday for a week, Monika fell behind in her work, but intentionally didn't commit anything, probably wanting to let Sina do it to avoid negative feedback again... When she came back from holiday I talked to Sina and changed a setting so that work sent to Monika from her account would still be flagged as hers after committing. I also logged into Monikas account (I have a habit of watching people enter passwords, I think I know those of all coworkers by now), and commited all the work Monika had marked as completed but not yet commited (the stuff she usually lets Sina commit). 

This didn't go unnoticed by the boss, and he angrily came asking why so much of Monika's tasks where flagged as Sinas and if that was the reason why Sina was always late with hers. It didn't end well for Monika (partly because of me and another coworker who said that we often saw Monika carry documents over to Sina) and she got a warning from the boss that she could forget a promotion and might lose her job if she does it again. Sina on the other hand got a raise.

TL;DR: A coworker was being a bitch and pushing her mistakes on someone else. I exposed her in a sneaky way and made sure she'll not become my boss to avoid future trouble with her.

AITA for doing this?