I have a friend who was pregnant the same time I was with my first child and we was due the same time which was Dec 12, she tried to hide it from everybody so she didn't eat right or take vitamins.. When she finally decided to tell someone she was already six months and was hardly showing and her daughter was really small.. Her daughter had so much problems before she was even born she ended up weighing 3 lbs and was born way to early.. I also know of another girl who never went to doc still partying and stuff like that and when her son was born he only weighed 2lbs an 6oz..
To be honest and this Is juss my opinion.... but that's a really big difference and I think she miscarried. But I can't judge, I don't know her or anythin' about what's goin' on but from what you said, sounds like a miscarriage to me and she might be lying. And If she Is tellin' the truth, then when the baby Is born, he'll be around 5 months Instead of full term and will have major complications and has a slim chance of surviving as the lungs won't properly be developed since the "point of viability" Is usually considered to be 24 weeks.