Allie, Harry, and Ron squeezed together under the Invisibility Cloak, reaching the fork in the corridor that led to the Slytherin and Gryffindor common rooms. Allie declined Harry's offer to walk her all the way to her common room; she had absolute confidence in her potions, and Mr. Filch would certainly never patrol that particular path.
Besides...
"Brother, just look at the state Ron is in. You guys head back first." Allie simply could not ignore Ron's ashen face.
Chapter 83