My wife bought a rather expensive dress a few weeks back and wore it
to a wedding at the weekend, but even before the party started, the
stitching of the hem started falling out,in at least three places!

To cut a long story short, I told her to take it back to the shop she
bought it and request they fix it (a standard obligation for shops
under UK law, for instance) but she said something about how they'd
tell her she should have checked properly before buying, etc, etc.

What does Japanese law say about this kind of issue? I find it
difficult to believe that it would be "cave emptor" as my BYJW
implied.

Ken