Alicia,

The one thought that jumped into my head when I read your description was
"GET RID OF THE UNDERWIRE!"  (Not yelling, just really emphasizing!)  IMHO,
regardless of how long she's been wearing the underwire bra, this still could
be the culprit.  There may have been a subtle shift in the wire that puts
pressure on the duct system in just the 'wrong' spot.  A change small enough
that the mom may not have even noticed it.
I refuse to wear underwire bras while lactating for just that reason.    I
would reccomend that she either go without a bra for several days or change
to a non-underwire and see what happens.  And of course, follow up with a Dr.
 since the underwire may not be the cause.

Jay