Unless they are doing implanted or injected radiation, I think the OB is being a bit extreme. Most radiation treatment for cancer involves using a radiation beam to target the cancer. It is like an x-ray but more focused and there is no radiation left after the treatment. Some newer treatments involve implanting radiaoactive pellets for a time period and in that case, there may be some issue. Also, there are treatments (not necessarily for BC, I'm thinking thyroid) where radioactive substances are given via IV and can last in the body.
I personally would ask her mothers' radiation oncologist, the doctor ordering the treatment, if there is any risk of radiation exposure after the fact. These things are tightly controlled and they should know more than the OB.
Mama to three boys ('03, '05, '07)