By posting his assignments, even after the due dates, he may be influencing his fellow students in the follow on assignments.
I really don't see anything wring with influencing your felllow students. Learning from each other is as valuable (maybe even more valuable) than learning from a book or teacher.
Assuming that he did not do things the wrong way. But even if he did things the wrong way you could discuss what is wrong about his implementation and why it is wrong.
Those discussions can be very helpful (whether you're explaining or trying to understand why it is wrong)
I think that the focus should not be on secrecy, it should be on openness. In the end every student is(should) there be there because they want to learn something and they should realize
that by copying the work of someone they are going to get in trouble later on. Students will talk to each other and assignments that change very little will be shared anyway, so you'll might as
well be open about it and motivate the students to do their own work and stimulate them to work together. That way you'll develop the skills to work together; have more fun because you can share
your frustration of not knowing how to handle a certain problem (and finding out that your friends can't either) and you'll learn more because you're enjoying yourself.