Just rewatched the scene. What happened is Poussey was throwing up in the bathroom from being drunk and Vee, Black Cindy, and another woman we don't know were brushing their teeth/at the sinks. So Black Cindy was there. Poussey gets up and starts yelling at and hitting Vee, Cindy runs away, and then Suzanne comes from another shower stall it looks like and pulls Poussey off and starts beating her up at the direction of creepy looks from Vee. I remember watching it and thinking it was good Taystee wasn't there, only because it would have made her even worse. Though it does look bad that she heard about it and didn't reach out to Poussey. But I would really hope that if she had been there, she would've done something.
Just rewatched and Cindy didn't actually leave-she watched. But I think her flashback confirmed she definitely was not one of the more more sympathetic prisoners/can be selfish. I'm not surprised she watched.
Also, I noticed Taystee also developed this attitude of "well, we're all criminals anyway so it doesn't matter what we do because we're already in jail." While I don't think that's a good excuse at all, I think it's how she justified her actions.