As others have mentioned, this is all about intent. I'd suggest that he's pretty screwed if they do happen to notice his visa application when entering under the visa waiver program. They'll absolutely refuse entry and make him sit in a holding cell until they can find a flight back. I know folks who've had this happen and it's not fun.
As for linenbasket's comment about "the slate being wiped clean", I would be very careful with that. The process for applying for a green card for the spouse of a citizen is not a 100% slamdunk. One of the things that you have to fess up to is whether you've ever been denied entry. Will it stop him eventually getting a green card? Probably not. But it will make the whole application process a lot more unpleasant.
Right now, the green card waiting lists for spousal petitions are around 18 months in Northern California.
They definitely need to get themselves an immigration lawyer.