While your friend’s conclusion might be sound and noble (“to not
prevent a pregnancy that could be deadly is irresponsible”), the end (i.e., avoiding pregnancy) does not justify every potential means to that end. Saint Paul wrote, “And why not do evil that good may come? -- as some people slanderously charge us with saying. Their condemnation is just” (Rom 3:8). The Catechism of the Catholic Church
(1756) explains it this way:
It is therefore an error to judge the morality of human acts by considering only the intention that inspires them or the circumstances (environment, social pressure, duress or emergency, etc.) which supply their context. There are acts which, in and of themselves, independently of circumstances and intentions, are always gravely illicit by reason of their object; such as blasphemy and perjury, murder and adultery. One may not do evil so that good may result from it.
Contraception is always gravely illicit and, thus, may never be morally chosen as a means even to justify a noble end. The Catechism
(2370; quoting Humanae Vitae
Every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible is intrinsically evil.
So it seems to me that when a pregnancy would endanger the life of a woman she should abstain from intercourse since pregnancy is the direct result of intercourse. Your friend’s noble conclusion might be better stated, “to engage in intercourse that could be deadly is irresponsible”.