Odd pregnancy question? This is confusing.

My brother just got home from boot camp (marines) August 9, and his g/f apparently got pregnant 2 weeks before he left in may, which she says puts her at 3 months. Well, she says her baby is only developing at 6 weeks?? I have never heard of that, I have 3 kids of my own. My whole family is wondering if this is his kid, including him. They have split up a few times before and she ends up pregnant,( but loses it due to female problems she really does have), which makes me wonder if she can even get pregnant by him. She isn't a big girl, she's not showing at all, just a little chubby in the face and backside, she did get a little gut, but not a bump, more flabby. But the main question.. Can a person be a few months along but the baby isn't??