Eating coconut shells is a terrible idea because it could cause all sorts of problems in your body. For starters, they might irritate or even rapture your esophagus. They could perforate the stomach lining, not to mention the intestines since they cannot be digested.
Why would you want to eat coconut shells? The nutrients of the coconut are on the INSIDE and not in the shell. You very easily could rupture your stomach linings, esophagus, your intestines and your bowels.