Found it. Actually, I should have found it ages ago and would have if I'd installed the game patch sooner. Apparently there was a bug in the original game that made it difficult/impossible to trigger that particular series of events, according to the notes in the last patch, which I finally downloaded a couple of days ago.

...I'm hoping the 'hardening' bit happens fairly soon because currently what I'm getting is her meebling over enjoying killing Marjolaine. Followed by a Greta Garbo-esque "I want to be alone". But then, given That Vision Thing and the her erstwhile career, I suppose she's going to be a drama queen, no?