followup: yes, this only occurs while coasting, I observed her while she road and this had no effect on her pedaling.
coming clean: she does have a screwed up derailleur from two prior accidents. The jockey-cage is bent as are the jockey wheels.
so much for wishful thinking eh?
The cogset is a year old, so no dice on cog wear, the chain is completely new. this has happened before and after the chain was replaced.
who knows then.