As someone mentioned in the other topic, have you thought about simply swapping out her birth control with vitamins? Instead of trying to talk her into the whole pregnancy situation. It might scare her off if you mention it too much. Switch the pills and she will have no idea that her pussy is preparing for an impregnation by the ex-boyfriend.

Just let her enjoy his dick, let them fuck like rabbits, and let him creampie her pussy multiple times per day. She would have no idea that her pussy is unprotected+fertile. The only thing that she may notice would be that she's super horny all the time, and that her pussy is wet all day long (Ovulating!!). She's gonna take it bareback like a good girl, she's gonna BEG him to cum inside her pussy, without ever knowing that they're making babies together.