I think it depends on the type of radiation your father-in-law is receiving. For example, if your father-in-law is receiving external-bean radiation, it will not make him radioactive. If he is receiving temporary brachytherapy treatments, he will be radioactive only when the material is within his body. There are some types of systemic radiation therapy that may temporarily make the patient’s bodily fluids (e.g., sweat, alive, urine etc) emit low levels of radiation. In such cases, the patient will be advised to avoid contact with children and pregnant ladies for a certain period of time. It is best to check with your father-in-law’s doctor for his/her advice.
As Hui Qun has mentioned, it's best to check what radiation theraphy your father-in-law is receiving and seek advise from his doctor instead as we may not be advising the right thing to do.
thanks