User blog comment:IamJakuhoRaikoben/Chapter 463: Black Carpet, Review./@comment-1776824-20151123213644/@comment-7418318-20151123235617

And what else is she supposed to do instead?

Rush the entire guild after Natsu?

The plan was to defeat the other three flanks and then attack the main force with a concentrated effort. If they just rush into Zeref's force right now, they'll lose, because they are too far outnumbered and at the same time the flanks might very well lose due to not getting support.

It might be harsh, but they can't afford to Natsu, no matter if his trumpcard is worth anything. If it isn't he'll die and there's nothing they can do about it.

NOT going after Natsu is the right tactical decision and also the decision that will have less of her friends die.

And who knows, maybe Mavis even has some inkling about Zeref not killing Natsu for one reason or the other. If she can be reasonably sure that Natsu will only be captured if he loses, that makes an ill-fated rescue attempts even less adivisable.