Using a condom is objectively evil. Yes.
Taking the life of a fetus is objectively evil as well. But, taking that life is permitted under the principle of double effect, when that life is taken as a result of some other action, such as removing a diseased womb.
I see this as little different. Using the condom will save her life. That it prevents conception is a secondary effect. That is not the primary purpose of using the condom; saving the woman's life is.