She was clearly against it and wanted to stop him. Then of course he throws the line "Worst Girl in the World" line at her and reminded her of how she should be selfish and live for herself (This is a personal peeve of mine because now it feels like Eren's using Ymir's promise against Historia and now she's doing it more for Eren's sake than her own, but I digress). Now I don't know what she could have done about Eren, but I'm sure she would have done something. But now she has to live with the fact she allowed so many people in the outside world to die. Now he did say he could wipe her memories if she's not happy about this. But even this, meaning she did something she couldn't live with so the answer is to wipe her memories doesn't scream "Live a life you can be proud of" to me.

Louder for the people in back