Judy,
I had a client recently who lactated sucessfully on only one breast, baby
gained beautifully.  (She had had some ducts removed for intraductal papilloma
and had the sensory nerve severed by the surgeon.)
        Lopsidedness is more pronounced in the early weeks, after that mom
might be able to even out her figure by using a couple of washable cotton
nursing pads in her bra on the non-lactating side.
        Is her nipple very sore on the "non-producing" breast?  If there is
truly no milk, the nipple would be very sore due to friction.  It is not at
all abnormal for milk production to be different in each breast. There is no
reason to discontinue nursing on that breast if it is all functional and there
is no nipple soreness. (IMHO)
Catherine Watson Genna, IBCLC