They were only a couple hours from heading to The Den, and this would be the first night that Hermione spent with Remus during the full moon. He had taken his Wolfsbane, and him and Sirius had thought of every precaution imaginable to ensure her safety. The transformation would happen while Hermione stayed inside the cottage, and Bill, Remus, and Sirius had spent the night before warding the house against entry from the outside. They could still use the Floo to get in, but once you walked out the door, no one was able to return inside. If Sirius felt that Remus was in his right mind, and Hermione wouldn’t be in danger, he’d release the wards and she could join them.