Your boyfriend's chances of being the dad depend on a few things.
First of all, he cheated and now is telling you that he slept with her from January 27 through February 3. If he was cheating and hiding it from you, what makes him a paragon of truthfulness about the dates now?
But let's say he was telling you the truth about the dates, then the next question is whether her period's expected due date is correct. Are her cycles super regular? Such that she knows for absolutely sure the next one was due February 1 and no mistake? (I assume she has no reason to want your boyfriend to be the dad in preference to her boyfriend?)
Anyway, the next question is to find out if she's pregnant or not. Women miss their periods for other reasons besides pregnancy, and if she has fertility issues she might just be having a wonky cycle.
If it is true that he didn't sleep with her until January 27, and if it is entirely correct that her period should have come February 1, then she should have ovulated a week or more earlier than January 27, pointing to her boyfriend as the father. But saying that sex on January 27 would be too late to produce a baby might be inaccurate, since if her cycles are irregular or if your bf is fibbing about when they began having sex, there is no way to judge.
If she turns out to be pregnant, your boyfriend could look into getting a prenatal DNA test from Ravgen or the DDC. It costs a lot more than a DNA test after the baby is born, but it might be worth it to know earlier so he can prepare. But until you know if she is really pregnant, this is all pretty theoretical.