Drake was in a bad mood. Since lunch, he has been in one. When he confronted Shirley he did not get a clear answer and now, he felt that his head would explode with all the thinking he was doing. He was trying to figure out what and where it went wrong but failed to deduce any sane outcome.
He had no heart to play guide to these people, especially on an empty stomach. He realized later that he had forgotten to eat. So when they departed from the café he asked his friend, Darren to take over while he stayed behind. Darren did not want to go but his persuasion power was too good. Basically, he had pleaded until he agreed and also promised to join them, soon, once he was full.
Yeah, so that's how he managed to let his friend take his place.
