The thing is if you try too hard than they get suspicious.
What you are selling is that you understand what is happening to his material and that you would not pursue the recovery in the way he does.
In 1995 Roland Loewen wrote an excellent book called "Small scale refining of Jewelers waste" an excellent book that all on this forum interested in processing material generated by jewelers should read. In his book he discusses melting sweeps in a crucible in a gas melter.
He details a specific flux mixture and a fixture made specifically to determine the relative fluidity of the flux. bottom line is he did not use a collector. This book set the medium sized refiners in the industry to buzzing and I know of more than one house who tried this method.
He mentions the burnt sweep should be mixed with the flux and slowly added to the crucible until the entire mass is molten. This alone, adding until it is all in and molten, often takes well over an hour. Then he suggests keeping it molten for a minimum of 1 hour to allow all of the particles to come together to form a bar. So with a small jewelers lot, maybe 20 pounds, incineration can take an hour or more, then fluxing and melting another 2 hours minimum. By the time you get your button and leave I'm guessing it is a good 4 hours. (for 20 pounds) How long is your jeweler waiting for this process? An important fact to know.
I admit I tried this method as well. For some sweeps it worked for others it didn't. Having assayed all sweeps before processing it was easy to see this method worked better for the sweeps assaying 7 or 8% precious metals per pound. These were the best of the best sweeps, and usually the exception rather than the rule. The lower the metal content, the worse the process worked. At that time I had access to a Cutler Rotary Furnace and I also conducted sweeps melts using Loewen's flux formula plus copper as a collector. The difference was night and day, the copper collector, benefiting from the rotating of the furnace and the reducing flame, effectively removed 98% of the metal.
If they are adding copper, they must some-how roll their crucible to allow the swirling pool of molten copper to collect the small particles of gold in the sweep. It is easy to tell if copper was added, the bar will be pink and much bigger than expected. What color is your jewelers bar?
I could of made a good living off of the gold left behind by straight melting the sweeps in a fixed crucible without copper as a collector.
So 3 questions for your jeweler.
How long did the incineration and melt take?
Did the operator rock or remove and swirl the crucible routinely?
Was the bar pink?
If the time was less than 4 hours, the crucible wasn't swirled routinely and the bar wasn't pink, ask him to retain all of the slag from the process and offer to have it assayed for him. There is only one way for the guy burning and melting the sweeps to make money, (Other than charging $1000 for the service) and it is in the residues.
