She didn't trust Draco. Not one bit. If he could see an advantage in betraying her, he would. Even though she knew all of that, the entire story of Rodolphus" treachery came tumbling out of her mouth. Though she didn't tell him the name of the memory charms expert or that her brother-in-law helped bring him, she told him every other disgusting detail. He knew about Rodolphus blocking her memories to hide their sexual and possibly romantic relationship, tripping her down the stairs while she was pregnant, and how he cursed her to make her attack her husband because he wanted Antonin dead and her son to hate her. She told him every single horrifying fact that she knew. He needed to be aware exactly what the man he was following was capable of. If he was horrified, he was skilled enough to keep his true thoughts off of his face.