I don't consider myself an expert, but by analyzing this hand it was clear that there was a chance he had something like that. What we have to focus on here is not only what range we put our opponent on, but also, as many poker books
tell us all the time, we need to focus on what our opponent thinks we have. That is the key in this hand. IMO this is a pretty decent player, he made a great move because he perfectly understood what bombjack had and gave himself a real good chance to get away with it.
If we look carefully at HH this player called a $2 PF raise and after that he called again when BJ raised it to $10. Now, when the flop comes and pot is $40.75 nastydog leads out with a miserable $3 bet to which BJ just calls. Here is where you have to understand that this player will narrow the range where he puts BJ on and probably pinpointed him either on a PP where he was scared of the A on the board, or he could be slowplaying a big hand.
Then came the scare card which was a perfect opportunity for him to take advantage of. That huge $20 bet pretty much shrinks the chance he has an A, unless it was the A of diamonds or a strong diamond kicker as was pointed out. His bet more likely means a flush or a hand worst than BJ's. It was pointed out by BJ that he was loose preflop and he only made calls preflop in this hand, he never raised so his range can be wide.
I obviously don't blame BJ for folding there, because a call or shove is a risky move but if you analyze it carefully you can see how this is not
a clear fold. It's not all about the range you put your opponent on, its also about what he thinks you have.