You're going to pick up a dry wine yeast (as in, one for making a dry wine; doesn't really matter if it's liquid or powder). It won't be picky about ABV or temperature, and it'll take that sucker down as far as it'll go.
You're also going to want some honey, a gallon jug, some yeast nutrient (DAP should be fine), and another carboy. Bring your hydrometer and autosiphon from home, and some starsan, and maybe a scale for weighing out honey.
So, first thing you're gonna do is find some space inside your friend's house for the five or six existing carboys. If midwest cellar temperatures aren't cold enough to send your yeast into hibernation during the summer, they sure as heck will be in January!
Using your autoshipon, starsan, and spare carboy, you're gonna rack the mead off the lees – after six months, as my dearly departed great grandma would say, that yeast is dead, dead, dead. This will work like one of those little square puzzles with one spot missing – you'll rack the first carboy of mead into the new carboy, dump the lees, sanitize, rack the second carboy into the now-empty first carboy, etc.
Now that they're racked, you'll give those carboys about a week to warm up to house temperatures. As a bonus, that near-freezing mead should outgas a bunch of CO2 as it warms up 20 or 30 degrees, purging the carboys of oxygen. And, your friend will not be idle during this week...
...because, he will be keeping an eye on the big ol' yeast starter you mixed in the gallon jug you brought. Make about 3/4 gallon of 1.040 "mead" with the honey and yeast nutrient you brought, pitch your wine yeast, treat as you would any yeast starter. Proper starter technique, and the appropriate amounts of honey and yeast nutrient, should be readily google-able, if you don't already know 'em off the top of your head.
OK, is that first big starter fermented out? Should be after 48 hours, though you can give it 72 if you wanna be doubleplus sure. Now, your friend is gonna make a giant freakin' starter in the last carboy (see? I didn't have you pick up another carboy just to make it easier to rack off the lees...). Two and a half gallons of properly-nutriented 1.040 starter mead should do it, just dump the whole 3/4 gallon starter in there, spent starter mead and all, you've got plenty of room.
Once that starter is done, he's gonna take it down to the cellar to cool crash for a couple days – you'll have almost three gallons of probably-rank, certrainly-weak 1.040 mead in there, you don't wanna dump that all into the batch you're trying to save. After a couple days, he should have a reasonably-compact yeast cake; he can pour off most of (but not all!) the starter mead, until there's only about as much mead as there is yeast, then resuspend the yeast into a slurry, and apportion it evenly into the now-warmed-up carboys of stuck mead.
HomebrewDad says you'll have about 900 billion cells just rarin' to go by this point, if they can't un-stick this ferment, nothing can.
