How to deal with two students who potentially colluded when one is unavailable?
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.