Given only the information in the post it seems prudent to assume that both A and B are roughly equivelant in skill level, and that neither have additional research on the matter (why bother comparing the efforts of A and B if you're allowed to fudge in that B is a better programmer and so knows he can do it faster, or you fudge in that B already knows of a partial solution and so knows he can do it faster with that as a basis). All we know is that both heard and accepted the assignment, that A jumped right into research and design and ignored the possibility of pre-existing solutions and wrote it in three days. B blew it off completely until Monday without looking at anything. Presuming relatively equal skill sets it would take B roughly three days to do it on his own just as for A. B banked everything on finding a suitable existing solution to modify at the last minute. A fairly sloppy attitude in my opinion, and something that is likely to be harder to correct than teaching A the value of prior art.

On the other hand, I happen to agree with both chromatic and clintp that Friday 4pm assignments with Tuesday deadlines is first and foremost a problem in management. Which is why I said that both A and B should be considering other employment.