The error here is that Broome passes over to another metric without seeming to notice. From the situation where she lives and enjoys life, it would be worse for her to have never lived. That is, now that she can consider anything, she can consider a world in which she does not exist as less preferable. In the situation in which she never lived and can consider nothing, she cannot consider it worse that she never lived.

FWIW, I think this misinterprets Broome's argument.

As I understand it, the argument is not that she cannot prefer anything if she does not exist. The argument is that if she does not exist, her well-being is undefined - and that it is consequently impossible to compare this undefined well-being to whatever well-being she may have if she does exist. The latter point does not depend on whether we view things from the perspective where she lives and enjoys life or not.