Evacuate.
Ok, so, here's the way I see it, Necromancers are usually pretty dubious, now I'm sure there's loads of nice necromancers out there, but the general image is tarnished, you know? Even if we have full assurance they're not at all evil and planning to wander off, how do we know the enemy aren't capable of pulling some trick out of their arses, like some kind of anti-necromancy mummified cheese-slab? These things are more common than you'd think!
But, hey, let's assume that we've somehow beaten off the Menacing Hobgoblins with or without the necromancers help, what then? Will the necromancer stay? I wouldn't expect it. Even so, we aren't able to defend ourselves against the inevitable next attack, sure, the Menacing Hobgoblins are defeated, but now the Irate Kobolds are capitalizing on the sudden power vacuum to become the dominant force in the region, if we didn't accept the necromancers help,we've lost most of our townsfolk, and couldn't survive another attack. If we accept the necromancer help, then they leave, we're back at square one, and if the necromancer stays... well, there's a finite number of serviceable corpses in the graveyard, and only one way to get more of them.
It's a no-win situation, cutting your losses and getting out with what you can is the best option here. (Well, actually, no, the best option is appealing to the Lord of the Land for military aid, you know, that guy you pay Tithes to for exactly this reason? Surprised that wasn't one of the options tbh!)
...of course, it's kind of a moot point, I find I'm usually the one doing the raidin' and lootin' in these sorts of situations. Remember folks, pillage then burn!