Bernard ignores the conversation because he knows his fate would be sealed even more if he spoke up.
Step-by-step explanation:
In the conversation between Mond, John and Helmholtz he knows that Mond will have the upper hand and anything that Bernard might say about what he was willing to do against the new society by reading the texts and organizing people who fail even further.
So he stays silent and only talks up to beg Mond to reduce his sentence for his crime of doing it but his friends had until them pretty much spilled the beans and had engaged in a discussion about the things they were doing.