Just dump the case, clean the fretboard, oil it, and live with it. It's not like even the reccurance of a little mildew is going to hurt the function -- you already know it plays. And the mildew's not going be on any of the finished surfaces, so you won't be getting it on you.

Eventually, when exposed to open air enough, it shouldn't be a problem.

Before spending lotsa money on it though I'd sure give the neck a good look. If it's moist enough to grow mildew it's moist enough to have some serious wood bending going on as well. You don't want a guitar, much less a 12 string, with a twisted neck.

Make sure you get a photo of you sitting crosslegged on the floor with it -- like Gordon Lightfoot.

...Sundown you better take care if I find you've been creepin' 'round my back stairs