Original Message

'LIFR...everywhere else was reporting CLEAR, and the pilot stated he continued to descend  into the "fog"'

Sometimes you can see down through the fog perfectly well if it's a shallow enough bank. You're only looking down through, say- 100 feet. Then, as you descend into the bank you're trying to see at a slant range that's, what? About 400 feet? And all you see is fog, except straight down....

No doubt continuing the descent was a mistake. Go around, abort the approach if any part of the developing situation is unsatisfactory.