What to do/

Oh,

And as far as the equipment, you can get an idea of the value of each piece, by seeing what others are asking on some DJ specific sites and or EBay.

All the best.

Most DJ’s are hard on records and equipment. Count the number of records and figure a dollar apiece for them and try to sell as one big lot. But don’t expect to get a thousand dollars for a thousand albums.
At a garage sale about six years ago, I bought roughly 975 CD’s for $150. I kept about a hundred to fill in voids in my collection and sold the 875 for a hundred dollars.

YRMV

You will spend an ungodly amount of time selling them piecemeal.  That is your main consideration.

If you're located anywhere near the Chicago area there's a record store in Elk Grove Village called the Record Warehouse. 

A good friend and I sold the bulk of our collections there, and we were both shocked at what they offered us.

They specialize in rock and roll.

Sell the whole collection to 1 buyer. You’ll be lucky to get 1$ each. Let the buyer sort them out. Most records aren’t worth much more than that wholesale. 

Make a Discogs account. Add all the records with their condition. Put whole collection for sale.

Bad part of a D.J.'s collection is the abuse that most of records have probably suffered. All the handling and back cueing (depending on what he played) may have hurt them.

Sell piece by piece on eBay or Discogs or something like that, which is a great deal of work for a nice-sized collection, or sell in bulk to a store or person, and they'd receive nowhere near as much, but is much more easily done.... I don't know what you'd consider 'getting taken', but likely won't get anywhere near as much as they'd like to think. I wish him luck.