This has never happened to me before and I there and then assured
her that the stone would be replaced at my cost.
you might have been better to ask her if you can inspect the ring
before making the commitment to fix it free.
If the stone fell out because of unusual wear over which you have no
control you could explain that to the client, diplomatically inform
her that is her unfortunate if inadvertent doing but that none the
less you would be willing to fix the ring at a reduced price because
you value her as a customer.
If the problem was some defect in design or execution then indeed
honor your implied warrantee with no fuss.
Unusual wear markings suggest abuse by the customer.
If I'm understanding the setting correctly, a few things to look for
are if a corner of the baguette was exposed and if there was
sufficient coverage of the metal over the ends of the stone. Also
there should be enough metal to tie the components together without
flexing. If those items were indeed properly done, point them out to
her, explain that gold is not tool steel and make whatever
accommodation you deem appropriate in the context of customer
But sometimes you just eat it even when its not your fault.