From the Daily Mail article linked to above:
Earlier today, a passenger on the outbound flight from Dusseldorf to Barcelona, which is believed to have had Lubitz on board, questioned why the co-pilot didn't down that flight instead.
Michael, 45, said he saw the pilot leave the cockpit during the 6.45am flight for a toilet break.
He told Bild: 'I sat in the fifth row. I could see the front of the plane. The toilet behind the cockpit was apparently out of order, I could see the red light flashing throughout the flight.
'A man over 40, probably the pilot, in a grey V neck sweater came through the curtain. He went through the plane probably to the rear toilet.
'After an estimated three to five minutes he came back and again opened the cockpit door.'
It seems unlikely that the forward loo was fixed during the turnaround, so the co-pilot would know that the loo break in the return flight would take longer than a few seconds. Also, this would tell him that the pilot did not have a personal embargo on visiting the toilet during a short flight.