Oh, he absolutely is as responsible as Monfriez, and that's why he went downhill after the incident.
I think he didn't speak up first because of shock, and also there may have been some thought about why Monfriez was doing this. It was to avoid getting in trouble, and Damon's character realized that he too would be in trouble if Walden survived to tell the tale of the mutiny. So he did have some hesitancy about speaking up for that reason, coupled with the fact that Monfriez was intimidating him. . .he hesitated, and then they napalmed everything, and then it was too late.
You must be the change you seek in the world. -- Gandhi
No way was he just as responsible. He was clearly about to speak up before Monfriez jumped in. The pilot nodded immediately. There was no time to say "he's lying." Even if he did, how the hell do you explain that I seconds while your pilot had to worry about enemy fire?
No, Monfriez says "she's dead" and they show a shot of him and Damon staring at each other. Damon had time to intervene before the pilot nodded, and especially before they actually napalmed everything. But Monfriez intimidated him and he made the choice to keep quiet.
You must be the change you seek in the world. -- Gandhi