This is an extraordinary notion for Mrs Gould to have, and I'm not sure how to interpret it. Perhaps she means that, with Montero victorious, all is lost anyway for her and Charles, therefore any attempt to salvage the situation is better. But this particular attempt -- further war -- obviously can and in fact does do a great deal of harm, which the altruist Mrs Gould would be blind not to see. Moreover, even if successful, the Separation plan amounts to withdrawing the support of San Tomé from the suffering people in the whole remainder of the Republic, a clear betrayal of her altruist ideals. Perhaps Mrs Gould means that the mere idea, as opposed to the action itself, can do no harm, but this would be uncharacteristically simplistic and morally evasive.

I'm forced to interpret the line like this: in her conversation with Charles, she learned that he was prepared to go "any length" in pursuit of his ideal; therefore he cannot be counted on to flee to safety with her. He will stay and likely they will both die. Therefore, Decoud's plan offers "safety" -- for her. It can do no further harm -- to her. Especially considering the ease with which she waives "right or wrong" here, I believe that in this line we are seeing one of the rare glimpses that reveals the profound self-centeredness underlying the altruist viewpoint.