User blog comment:Miskos3/Fairy Tail Chapter 326: Natsu vs. Rogue, Review/@comment-7854514-20130408155446/@comment-7418318-20130408161157

Would she really be willing to sacrifice 90% of the worlds population because the spirits would be harmed?

I mean even if they are somehow wiped out completely, wouldn't that still be better than millions of Humans dying?

As for when it was said, it was when Arcadios and Yukino explained the original Eclipse project to Lucy & Co shortly before they were arrested (Chapter 302). Though "it" being only the part of the boundary, not anything about spirits being harmed.