Lenta Johlitz worked with Christian B at a corner shop in Braunschweig, north Germany.
"There was this one situation where we sat in his living room and spoke about Maddie," Ms Johlitz told ITV News on Tuesday.
"He suddenly got very aggressive, nervous and agitated.
"He said very loudly the child is dead.
"After all these years you wouldn’t find such a corpse anyway.
"That was the only situation where I look back and ask myself: Why did he say that back then?"
Her comments came after a 75-year-old man whose daughter went missing in Europe 19 years ago wants German police to question Christian B about her whereabouts.
Student nurse Louise Kerton was last seen in in July 2001, where she was supposed to be catching a train from the city of Aachen near the border with Holland and Belgium before taking a ferry to Dover.
Speaking to the PA news agency on Tuesday, he said of the latest news: “It just confirms that the family have been right to keep persisting, there was something to find out there.
“Certainly the German police seemed confident they are onto something there.
“It gives one some sort of hope that the truth is out there to be found.”