This is an admittedly extreme interpretation, but I see a very subtle thread in Mrs Gould's dialogue that shows a physical attraction to Decoud. Remember, she has just come from her interview with Charles in which he gave her only a kiss of the hand, amid hints that she wanted more. Here with Decoud, she is in turn "eager," wide-eyed and smiling, unsteady on her feet, and "amiably" welcoming. When he finally confesses his passion for Antonia, she listens "unflinchingly," as if let down. These reactions are heavily cloaked under her natural eagerness to hear Decoud's news, but if there, her attraction would increase her receptiveness to Decoud's (quite brutally framed) demand to keep Charles in the dark.