In Shulchan Aruch,(CH. M. 232:19 ) it states
Minhag is Mevatel (nullifies) the Halacha ( מקח טעות)
Today we can't claim מקח טעות
(The buyer is stuck with the spoiled eggs and can't claim מקח טעות )
Harav Elyashiv ruled this Halacha/Minhag is only applicable when
someone sells for profit since the Minhag in the business world
it is acceptable to claim a final sale - Caveat emptor.
However a woman borrowing eggs since there is no profit, then
the original Halacha of מקח טעות kicks in and it is applicable.
Therefore she hasn't acquired the egg and is not responsible
to pay for it*.
The Shevet Halevi (10:133) writes "A woman who borrowed a dozen
eggs and two of them had bloodspots, She might not be
allowed to return a dozen eggs because there might be
a problem with Ribis (D'Rabonon)
*She is also not responsible for destroying the egg since it has no value