Mr. Koophuis had met Mr. Frank in Amsterdam in 1923, when they both had business dealings there. This association continued intermittently until 1933, when the Franks moved to Amsterdam and the business relationship and personal friendship between the two men grew. In 1941, Koophuis took over Mr. Frank's place in the Travis company — otherwise, the firm would have been confiscated or liquidated as a Jewish business. It was Koophuis, together with Kraler, who proposed that the Franks use the back of the business building as a refuge. They helped the Franks move furniture and household items there, by stealth and at night, in order to avoid detection.
When a postcard reached the Franks in 1942, ordering Margot to report to the reception center at the Westerbork camp, everyone knew that the time to act had finally come. Mr. Koophuis was instrumental in ensuring that the secret of the group in hiding was kept, even though this raised many technical difficulties, particularly when the ownership of the building changed hands and the personnel in the warehouse also changed. Food had to be obtained for the group in hiding and paid for, extra food ration stamps had to be obtained, and in many cases, this aroused people's suspicions. Nevertheless, the baker, the vegetable man, and most of the other people with whom Koophuis had dealings, did not ask embarrassing questions; they simply cooperated in silence.
Mr. Koophuis has described the arrest by the Gestapo in the following words: "It was a Friday, and a fine August day. The sun was shining; we were working in the big office, Miep, Elli, and myself, and in the warehouse below us the spice mills were rumbling."
While Mr. Kraler accompanied the police in their search of the building, Mr. Koophuis and the two girls were ordered to remain at their desks. His first concern was to protect the two girls, and he told them to leave the building and insist that they had been unaware of what was going on, if asked. He was taken with the others to Gestapo headquarters, but largely because of his presence of mind, Miep and Elli were not taken too.
As they left the building, Mr. Koophuis relates: "I was the first to step out on the street. People were standing around on the sidewalk, staring as if there had been a traffic accident. They all looked stunned. I was also the first to get into the van and sat down way up in front, behind the driver."
As they waited in the cell at Gestapo headquarters, Mr. Frank told Mr. Koophuis how bad he felt that this had happened to them. Mr. Koophuis replied: "Don't give it another thought. It was up to me, and I wouldn't have done it differently." Koophuis and Kraler did not talk to their captors, who did not invest very much effort in forcing them to do so.
Fortunately, an international welfare organization intervened on behalf of Koophuis, pointing out that he was ill. He was released for medical care after a few weeks of imprisonment, and then he returned to Amsterdam.