Hoping in a recovery, collection to save!

I hate to say this but I think your friend's collection is too far damaged, especially if it was river water or water from a flash flood that flooded his garage. Even if you could get the covers unstuck from the vinyl, the whole lot is still going to smell and likely develop mold. The only suggestion I have is to maybe try lightly rewetting some of the less-damaged covers and gently peeling them from the vinyl, then giving the vinyl a really good clean before putting it into a generic sleeve. The original cover will likely have to be tossed. That's what a friend of mine did when his basement flooded and damaged his collection. Most of it he ended up tossing, but a few he was able to wet the cover down a bit and then peel it from the vinyl.

If he's a former DJ then many of the sleeves should just be plain black generic sleeve they can purchase again.

I'm sure they can save them (well, some) - just look online and filter the information, there's lots of info out there.

Will some be lost forever? Really depends on the water, severity of the damage.

If they're in Sardinia, the baking heat should dry them out in quite literally a few days. After that, it's all about seeing what you can salvage.

Cerca online per più informazione (tipo Youtube) Qualche cosa si potra salvare.

If you can get them unstuck, for the worst of the records, turpentine is safe for cleaning vinyl and will take off pretty much any of the nastiest stuff you can imagine. It will stink to high heaven though, so requires multiple follow up cleanings with a drop or 2 of Dawn and rinsing with de-ionized water followed by a good airing out. I have cleaned some of the grubbiest, nastiest scary looking vinyl this way and the results were remarkable.

The sleeves are all goners, I assume. Nothing you can do about them once they have bad enough water damage and mold. If they are structurally sound but very dirty, as long as they are laminated, turpentine will work on those, too. The inside can, if not too horrible, be handled by propping open with a toothpick or something and giving a light spray of rubbing alcohol or vinegar (again with the residual smell) allowed to air dry. If still some smell persists, you can put some baking soda in there and leave it in the sun.

Also, if the sleeve doesn't have mold damage but is just wavy and you care enough you can look up some of the methods book preservationists use. I've not done them myself, but directions are out there. The craziest, but apparently standard practice is completely soaking it again (I think it was in boiling water) and pressing until dried (don't just follow my steps, look it up in case I'm missing something :)). Sounds kind of scary to me, and I always meant to try it with a throwaway spare sleeve, just to see how much better it can get.