If she wasn't careful, she would become one of his victims. Even the benefit of their fucked up friendship and years-long sexual relationship wouldn't save her from the locked room in Level Eleven if he suspected she was up to something to sabotage the regime. In order to be successful, to have Aberforth Dumbledore walk out of the bowels of the Ministry and for her to not take his place in his empty cell, she could not make it obvious what she was doing.