Setting aside the Chemical Technicalities for a moment, isn't this supposedly a Placer Deposit ? And how many people generally use Fire Assay to sample a Placer Deposit ? In fact, hasn't it been said by many an experienced Geologist NOT to rely on Fire Assay to sample Placer Gold ? In which case, why not just run it through a WashPlant and get as much as possible ? If there is concern that you are losing significant amounts of fine or flour Gold, then run the Sluice Cons through a Concentrator as suggested by Robert above - either a Knelson or an Icon. If you decide to go to the trouble / expense of setting up a more Technical " Circuit " and can get control of your Water through a Re-Circulating System, Floculants can be added to the water itself which apparently encourages the " Floating " Gold to sink and subsequently be captured / recovered.
Any of these non-standard Recovery methods require a lot of testing to achieve the ideal balance of Chemical vs Metal to reach the desired result of recovering more Gold. There would have to be a significant amount of Gold being lost in those Tails though to justify setting up anything more complicated than a standard Gravity system, particularly if it involves Chemical extraction. I don't think any of us ever expect to recover much more than 90 + % anyway.
Flint - If it's an extensive Deposit that justifies the expense and your friend has been unable to come up with any method of recovery, then it looks like he has no choice other than to do some serious Bulk Sampling through a purpose-built Pilot Plant under the guidance of a good Metallurgist. Best of luck with it and let us know the outcome in the event one of us experiences a similar problem.