I too have a large collection of LPs, something between 2500 and 3000. A general rule of thumb is to store media in an environment comfortable to humans. While I would be heartbroken at the thought of loosing them all, the situation here would force me to make difficult choices.
Time is money and any restoration project is going to require an enormous amount of time on your friend’s part. Unless he has a Beetles butcher cover or something equally rare the records are probably worth much less than his time.
The mold is the very first thing that must be dressed and there are methods that range from enzyme solutions to Clorox, mild dish soap or 90% isopropyl alcohol rinses along with the use of very soft brushes or microfiber cloths and lots of distilled water rinsing between steps.
The PVA glue trick is based on a commercial protect sold for a few years in the late 1970’s by the Empire Phono Cartage company called Disco-Film. It worked, and from what I’ve heard the glue trick does, too. I would think that one should probably do a pre-cleaning first but experiment and see what works best.
May I suggest that your friend should first make a database of what is in his collection and see if the music is available online. Youtube, Spotify and Archive.com should have most popular music published in the last 60 or 70 years. There are ways to download the music to your hard-drive or just log the URL for when you want to listen to the song. Concentrate on salvaging the disks that are not available online as that is an indicator of rarity.
Yes, an LP is going to sound better than an mp3 but not a damaged LP and I’ve found that by up-sampling an mp3 file and saving as a .wav file the music can sound as good as a CD and you loose those annoying Rice Krispies.
I’m sure he has digital copies or CD’s of a vast majority of what he has, but they were handed down from his father, who bought many of them new. Our group of friends can also appreciate the sound of vinyl over digital.