Well, she was in love with him. So, when Max broke the limits of friendship and wanted more, she couldn't stop. She didn't stop. Despite knowing it was wrong, she let him be. Despite knowing he can't be hers, she gave herself to him. Despite knowing Amayra is a sweet girl and doesn't deserve something as cruel and heartbreaking as getting cheated by her lover , she just couldn't stop him.
Rachel knew that the girl who helped her at her lowest was Amayra, still the passion , the lust, the love forced her to stoop too low even in her very own eyes to say nothing about Amayra.
As if luck had it or maybe bad luck, Zaco witnessed it all. Being a loyal and trustworthy aid of Amayra he reported it to her, even when his heart was shattering into pieces itself.