Sounds like that's what he did.
If half your grade is class participation (I've had several like that) and you turn in your A-level work then take a nap - VOILA! - your grade is cut in half.
The problem is, it doesn't sound like he did that. It sounds like he just took some random assignment and cut the grade based on something external to the assignment. No can do - assigned work should be graded on the criteria set forth for that assignment alone. If the teacher takes class participation and makes it an "assignment" unto itself, and the kid winds up failing the course because his participation wasn't up to snuff, so be it - I have no problem with that. But I don't think that's what he did, because if he had, we wouldn't be having this discussion in the first place ;)