How to deal with two students who potentially colluded when one is unavailable? We have two students in my department who handed in identical essays. As per our policy we scheduled independent meetings with both students to determine what happened. Going into these meetings our primary objective is to figure out which of the following 5 possibilities occurred:


The students worked jointly and wrote one essay
One student wrote the essay and knowingly allowed the other student to copy the essay
One student wrote the essay and the other student copied their essay without the first student's knowledge
The students worked independently and managed to write word for word identical essays (this seems unlikely)
The students independently copied the same source


One student is on medical leave and was therefore unable to attend the meeting. The student who attended the meeting showed us drafts of previous essays which have convinced us that neither (4) nor (5) occurred. Further, when we exclude the other student's essay, TurnItIn doesn't find anything of concern. We also are confident that the student who attended the meeting did not copy the essay from the other student. This leaves us with possibilities (1), (2), and (3).

Our policy states that both students would be punished for academic misconducted if either (1) or (2) occurred and only the other student would be punished if (3) occurred. It seems without talking to the other student we cannot really come to a fair conclusion. Without a decision the student who attended the meeting will have neither passed or failed the class and therefore is not eligible to retake the class nor take any class which has the class in question as a prerequisite. It is unlikely the student on medical leave will be back in time to make a timely decision. How should we proceed?
Not letting this student move forward at this point would be punishment. If there is a chance that you will find the student innocent of any wrongdoing, it seems like the injustice of punishing them for something they did not do is  worse than letting them get away with something they did. 

Make it clear to the student that they will pass the class given the lack of evidence against them but that you will continue to evaluate the situation. Make sure they know that if you conclude later that they have colluded, you will update their grade to a failing one and force them to retake the class.

The only real negative consequence of this is that the student might be able to take classes that require this class as a prerequisite before they are forced to go back and take the original class again. That's not ideal, but it doesn't really seem so bad.