"He would've left her knocked up" means that "he" in this case would have left the girl pregnant or knocked up, had the relationship been allowed to get to that point. The second one means that he would have left the girl after having physically impregnated her, implying that he is the kind of person to leave when the situation becomes complicated with something as serious as pregnancy. The difference is that the first one means that he would have left her, if he were allowed to, pregnant, meaning that he would leave after having unprotected Sex with her once, while the second means that they would be having sex more than once, but if she got pregnant, he would leave. I hope this helps. The difference is very slight.
@tjstkdn Almost. In both situations, by using "would" you show that she is not actually pregnant, but in both situations, the girl becoming pregnant was a very possible future. In the first one, the way it's written implies that he is the kind of person who would've had sex with the girl and then left, most likely with the girl getting pregnant as a result. In the second one, because we added "after", it means that he would've left the girl if she had gotten pregnant, not simply because they simply had sex. In both situations though, because we used "would", we imply that whoever we are talking about is NOT pregnant at the moment, but she almost was. I hope this helps!!!