If the wrong person discovered Draco's treachery, he would be in deep trouble. Likely even dead. She didn't want to run the risk that he would be dragged into any bother for helping her. Even before he saved her life from the damned cottage she was almost positive that she already owed him her life. There were a few times while she was on the run that he saved her and he was instrumental in getting the ingredients for the polyjuice potion that saved Aberforth's life. She pushed away the inconvenient reminders that he'd also betrayed her when it was necessary to further his own gains. Their relationship was complicated to say the least.