Anne,
I have had several moms who nursed after breast surgery, without incident.  A
mom with one intact breast can make enough milk on that one breast to
adequately nourish the baby, there is no need to be paranoid about the weight
gain beyond the normal.  I would be concerned that this mom should breastfeed
to reduce her breast cancer risk.  I would hate to see her denied this for an
unfounded theory that she will be more susceptible to mastitis!  Even nursing
unilaterally would cause milk to be made initially in the unsuckled breast, so
she might as well try them both.
Catherine Watson Genna, IBCLC