They should certainly communicate the condom failure. Should they then be happy with "At least, we'll kill it in the next stage...?"
Non-sequitur. Are you comfortable with a method that deliberately terminates a successful zygote, after-the-fact? Each couple must deal with this question before the Lord, regardless of their church's position.
I am skeptical that those who would answer in the affirmative could really claim be pro-Life, though. It isn't about discipline. It is about maintaining a clear conscience.
I'm not American; while I'm instinctively against abortion, I sense a lot of overtones of US political culture and discourse in some of the "Pro-Life" discourse.
But in any case, my rhetorical question in response would be: Does a woman really have to argue that she has a "Pro-Life" badge because she doesn't use bc, and not using bc makes her supposedly politically sound?
(I appreciate your patience in discussing these matters; you see, outside the US the whole area is less politicized. For example, in Quebec, there were never any attempts to enforce Prohibition, with all its negative consequences, although again, I don't drink alcohol.)