Originally Posted by Severian
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.
Chances are that it is cog wear after all, per rmfnla's suggestion. Many casual cyclists spend most of their time riding around in the middle front ring/small rear cog or small front ring/small rear cog combination. Since the rate of cog wear is inversely proportional to the number of cog teeth, the chain can start skipping on the smallest cog after comparatively few riding miles. The cheapest fix (assuming this is the problem) is for her to stay away from the smallest cog unless she's in the big ring in front.
If she's had a couple of crashes and the rear derailleur is visibly bent, the derailleur hanger may have been knocked out of alignment. Having an experienced bike mechanic check and correct the dropout alignment is cheap insurance against the possibility of running the derailleur into the spokes.