If a breast cancer patient breast feeds her child will he be affected?

No. It is safe to breast feed after having breast cancer, although the affected breast may not produce milk following radiation therapy. Of course, any medication you take, including chemotherapy, may be passed to your child in the breast milk; therefore, your child's pediatrician needs to be aware of any medication you are taking before starting breastfeeding.
Depends on tx. In general patients being actively treated with chemotherapy should avoid breast feeding. Always best to check with your oncologist.