Yennefer was here for Jaskier and Geralt primarily, but that won't mean she won't interfere if she runs into someone else she knows. She may not know Sara super well but she thinks she knows her well enough to at least assume she wouldn't do something like this normal.

And the idea of Mammon stealing away someone else's agency makes her blood boil.

But if she says something out loud she'll alert the nurse.

So instead she whispers in Sara's head.

You don't have to do this. Fight it.