@Yishai explained the practical course one should take. I want to fill in some alternative Halachik views.
According to some Halachik authorities, one is allowed to sacrifice one's life to save another's. The mother may therefore choose to decline treatment in order to save the life of her unborn baby. This, of course, would be contingent on the child being expected to survive.
More likely, the actual case contained uncertainties. The treatments may or may not damage or kill the fetus. The mother may or may not be able to deliver the baby and then commence treatments (I know of such a case; both survived). The mother's probability of survival with treatments might be very low. To quote, "The only certainty about the future is that the future is uncertain."
In such a case, one might find room to enable the mother to choose her preferred course of action. It is accepted that one may risk their life in order to save another's. This is subject to extensive debate on the mitzvah of lo saamod al dam reyacha - do not stand idly while another is killed. While the authorities disagree, they all permit one to take a risk to save another's life. How else could we allow security guards, life guards, or a host of other potentially dangerous jobs? The authorities disagree on the degree of risk one may take for another. Some permit even a major risk if one so desires to take it.
Just to be totally clear, ALL halachik authorities permit the mother to save her life at the expense of the fetus, even in case where the danger to the mother is uncertain. And of course, one should consult with a competent Rabbinic authority before making such major decisions.