Originally Posted by OLDYELLR
This question comes up so frequently, it should be made a "sticky". All freewheels wobble to some extent when coasting.There's nothing wrong and it doesn't hurt anything. Here is an explanation:
"Freewheels and hubs are made as concentric as
machining reasonably permits. However, threads do not constitute a
good centering mechanism and since freewheels are made of concentric
components that are mounted on concentric threads, they can easily
acquire some wobble, that as you must have noticed, does not affect
performance at the slow rotational speeds of a bicycle. That is why
the bicycle industry can use these designs that high speed and
automotive assemblies cannot.
Jobst Brandt"
Sorry, but Jobst is just plain wrong. The wobble in a freewheel is from the aforementioned gap in the ball bearings. Unfortunately, this does not help the OP because we have established that his fiance's bike has a cassette.
Severian, I think the chain skip has nothing to do with the wobble, especially since the intersection scenario would involve lots of pedaling pressure but not high speed. I suggest you check for cog/chain wear.