How to recover 15g gold from 50 gallons of 30% Acid?
You could distill the waste easily enough, but the distillation gases will be rather nasty. You will also need a condenser to cool everything down again or the environmental penalties (and corrosion damage to the exhaust system) will be a lot more than the gold value.

Once you have the waste boiled down (literally) into a small flask you could use any of the known gold-extraction methods to get it out in elemental form. 

At current market prices you have about US$580 of gold in the drum. No idea how much you will spend in fuel and cooling water to recover it but it will be a substantial percentage. Why don't you just tell "Someone" that they can either pay for the gold they dumped or mind the still themselves over the weekend (and pay for the propane). This will also encourage others to pay more attention to what is in the beaker.