Val -
I can tell you from my own personal experience that my 4th child would only
nurse from one breast for almost three months.  I could occasionally trick him
into taking the other side at night when we were lying down together, so I
kept my milk supply up.  He did finally learn to take both sides and he is
still nursing well on at 2 1/2 years old.
Let the babe feed from just one side and let that sore side heal.  (Is the
baby using his tongue incorrectly on that sore side?  Birth injury on one
side?)  Since mom has an oversupply problem anyway, she shouldn't have any
trouble feeding from just one breast.  Staying on one breast may also increase
the baby's intake of hind milk and cut down on his gassiness.  Good luck with
that mom!
Cynthia D. Payne
LLL of Berkshire County Massachusetts