He was on the board. It's their job to know what's going on, but they just didn't care to look or maybe know how. The people on the board were there for prestige rather than serving any real function unfortunately.
Yup, sadly read of testimony of one of them that they weren't really sure why they were there (on the board). Of course, not all, indeed maybe most, board members are not technical experts. The board is there to set general corp strategy, not to do code review, and are generally working under the assumption that the executives are not lying to them; executives who are found to be lying to the board are generally rapidly dismissed.
So, yes, I have a hard time blaming the board members who were used as much as the investors. I'm not sure there were ever any technically proficient board members, which also leads me to the conclusion that this was a planned fraud by Holmes, Balwani, etc.