Man could have literally just broken up with her, or been upfront about the fact he didn't saw their relationship as serious or exclusive when Louise clearly thought otherwise.I don't think Jeremy even sees anything that potentially happens as a mistake -- getting freed of a psycho "girlfriend" is probably a blessing for him, if anything.
Jeremy kept his mouth shut because he was already able to get some with Louise and knew that the moment she realized what he was about that she would dump his ass (at least with how the characters were at the start of the game).
Also he does admit he "messed up" when chatting with Lena some time after Louise breaks up with him, and he also got his balls busted by the boys when the subject comes up in their group of friends ("it wasn't me"). You could argue Jeremy thinks he only messed up by getting caught rather than cheating in the first place, but I don't think he is on that level of sociopathy yet, just a bit shameless and trying to hide under the excuse that it was just a misunderstanding between him and Louise that he didn't correct in time.