Hermione was one of the few Death Eaters who treated the dangerous werewolf like an equal. It had been a gamble when she first decided to try to befriend him in hopes that she would have someone on her side. There was a great deal of politics involved in being a Death Eater. That was one of the very first lessons she learned on her own. Even though the werewolf terrified her and she wasn't completely over being upset about his treatment of her the night she was dragged to Malfoy Manor, she cultivated that relationship. All it took was just showing him a little bit of respect and not speaking down to him like the others and he was hers. She never once had to question his loyalty. If she asked, and she was tempted to ask more than a few times, he would even go so far as to murder Antonin. Every other Death Eater might have been terrified of what her husband was capable of, but Fenrir would dispose of him in a heartbeat if that was her desire. It was helpful to have him entirely on her side.