It wasn't Theo's fault that Draco knew what was happening. The excuse to get him up to the tower actually wasn't terrible, but when the entire day had gone by without much of anything happening for his birthday, Draco had become suspicious.
No, he'd figured there was a surprise party for a while before Theo suggested that they do something fun, to cheer him up, since he looked put out that everyone had forgotten his birthday. He had given his friend a very pointed look, and it hadn't been long before he'd gotten the confession out of him. A surprise party it was.
The surprise might have been ruined already, but Draco was in an excellent mood as he headed up to the tower with his friend. He opened the door, took one step into the dark room, spread out his hands and yelled, "Surprise!"