Well actually, Will thought that Aileen would have heard him say the password when he brought her up to his dorm a couple of nights ago. He wasn't that concerned that she found it out, there were plenty of Slytherins (and people from other houses, for that matter) who already knew how to get into the Hufflepuff common room, and vice versa.
"You could have asked," he offered. That was also true. She clearly had just not made the effort.