It was so easy for Malfoy to find Gwen alone. When she wasn't surrounded with her new friends she was sitting on the edge of the Forbidden Forest, gazing into the depths of darkness. She spent glorious amounts of time there, just looking at the interplay of shadows and light on the leaves of the trees.
Perhaps Gwen remembered that night she and Harry had stolen into the woods and was looking back into the past, or maybe she was searching for something in the darkness, hoping it would reveal itself and clear her mind of any confusion she might still have. She wasn't sure, but she thought something lay waiting within those woods. It called to her and Harry that night when they went running about reckless and free. It was still there, still waiting for her.
The darkness was seeping beyond the forest and Gwen realized the sun was setting. It was about time to head back to the castle. She stretched as she stood, a little sore from sitting so long. She didn't even see it coming.
Malfoy had his wand out, pointed at her and ready to do mischief. And while his hex was no where near any of the Unforgivable Curses it was bad enough. A 5th year Slytherin's extensive knowledge of the dark arts comes in mighty handy, he thought.
Gwen was thrashing about for hours afterward in the hospital wing, trying to think of a way to get him back while Madame Pomfrey tried to soothe her.