In the Mental realm, she had been dragged into, Rita was out of her depth. Everything was against her and that was the whole point.
Harlyn would not actually engage in a duel, both mental or physical with any random person and she only did this to teach Rita a lesson in manners.
Lo and behold, Rita grinned in the mental realm before opening her mind to external influence. The external influence was, of course, Hal, and it only took a brush of his mental pressure flowing through Rita for her to strike against the Empress' control and eject herself from the mental realm.
Harlyn recognized that mental signature to be Hal's and she frowned before striving to feel more of it by striking at Rita's mind again but Rita was protected by Hal's mental signature and it would take increased effort hinting at battle intentions for the Empress to have a chance to break through.