I wonder if this is like a fraud my wife and I spotted in a branch of a discount supermarket?
It turned out that the staff were running a scam. They would key the last item of the preceding shopper as the first item on the following shopper in the queue.
As nothing in the shop was much over a £1.00 or so, nobody would notice. They were not using itemised till receipts, so nobody would have noticed. And if there's an extra 50p on the bill, how would you know?
How they were found out was that my wife was doing the shopping of an elderly friend, I was doing our shopping. The last item on her shopping was a bottle of brandy, about £8.00.
Now when an item for £8.00 suddenly appeared on my bill, we went back to the store to complain. The manager went white, and handed the money over without demurring. Odd in itself.
It turned out that they were able to keep a running tab with their till system, and all of the money that was extra was kept and shared out by some of the staff.
It's possible that someone at the partner search engine runs a similar scam with every -say- 10th search. But they hit a problem in that this time they chose to use a very obscure term. With someone on the case as sharp eyed as you.
The only reason I can think that Yahoo would not seem to properly investigate such a problem is if they asked the partner search engine who would lie and tell Yahoo everything was fine.
Either that or Yahoo do not care once they have your client's money.
Of course, the above might be a load of crap, but someone defrauded your client and the people who should have investigated could not be arsed to do it. It does make you suspicious, doesn't it?