Sorry about your old hen! Did you notice if this eggshell was particularly brittle, or soft? I think sometimes the eggs of newbies can be a bit brittle, and they certainly get like that when they are getting towards the end of their laying days - and that's when my original girls started eating their own eggs, cos they were just so easy to break. 
When we got some new hens and they started laying it all stopped (their eggs weren't brittle). So I dunno the answer really cos you'd think that once they'd got a taste for it (and boy did they go for them and fast!!! 

 ) they'd do their best to break the harder shelled ones, but no! 
