Perhaps Bonner's biggest problem here is that he learned about the forged letters before the vote on global warming legislation, but didn't notify the affected lawmakers until after.
Here's what Bonner said in his testimony on that point:
While our immediate focus upon learning of this fraud was to reach out and apologize to the organizations whose names were used without authorization, on July 1, 2009, we contacted the offices of two of the three Members of Congress who received fabricated letters. After numerous attempts and the intervening Congressional recess, it was not until July 13, 2009, that one of our staff finally succeeded in directly speaking with Congressional staff for Rep. Perriello and Rep. Dahlkemper about this matter although it appears that Rep. Carney's office, which received one letter, was not contacted. In retrospect, we should have immediately contacted all three offices and apologized in person.
And he's now being grilled on that point by Chairman Ed Markey. "I should have personally sat there to make sure the message got through," Bonner admits.
Bonner claims he didn't know the date of the vote. But Markey seems to find that hard to believe, given the high-profile nature of the vote.
You can read Bonner's full testimony here