Maybe if Valmont was fighting back, no. But we're assuming he did fight back because of the knife. He could have just as easily been shocked by her and been too stunned to fight back! And if she had the right kind of weapon, she could have killed him while he was stunned.

Plus, if she and Valmont did make a promise to meet...she's the one who said no to the buddy system, but if Valmont trusted her, then he wouldn't have worried about it until she actually struck him...