Hi.. If your partner is breast feeding, then she would be naturally protected against getting pregnant.. This is called as lactational amenorrhea in medical terms.. This natural protection is confirmed to be active present, if she has not started her periods. But if she has started having her periods, which would indicate absence of this protective mechanism, then you may have to use the emergency pills to prevent pregnancy..
Tell your wife to consume "Unwanted 72" pill within 72hrs of intercourse.It contains Levonorgestrel which can act as emergency contraceptive.