“Damn that Geoff, this was a Geoff thing not a Jansie thing.” Why did Sophie say so?

Sophie said so because she was angry at Geoff for giving away the secret, which he was supposed to keep to himself and not disclose to anyone. She had told Geoff of her imaginary meeting with Danny Casey. She knew he would easily believe her and not say anything to anybody. Besides, Sophie was also upset on being confronted by Jansie, who, according to Sophie, was extremely nosey.