Diesel in bilge
Our boat came with the diesel in the bilge feature--no extra charge. We even helped top it up when we did the sea trial and found the leaking fuel filter that was the source. Anyway, hopefully this hypothetical person has disposed of the diesel and cleaned up the bilge as quickly as possible. The longer the smell hangs around the deeper it seems to get.
We wiped down all surfaces on the boat and sprayed them with some odor removers like Pure Ayre. That definitely helped. But the cushions were the final odor repository, and it's not just the covers, which are relatively easy to clean, but the billions of pores in the foam. I painstakingly soaked all foam in a product called Kids N Pets, squeezed them all out, and dried them. Really did the trick.