Can a baby disinegrate inside your womb at full term?

Full disintegration does not happen. If the baby died and she didn't do anything about it, then eventually the baby would start to disentegrate, and it would become macerated (breakdown due to long exposure to the amniotic fluid), but for it to completely disintegrate would take over a year, and the body would expel it long before that. Besides, if she was going to the doctor regularly, then the doctor would have noticed something wrong long before that. It is possible for absorption of a fetus to occur, but even so there would still be some remains, and the body would expel most of it as absorption only happens in cases of a miscarriage of 12 weeks or less. If the body does not eventually reject the fetus, eventually she will become ill with septicemia and may hemorrhage. She would if nothing else be suffering from very severe cramps as the body tried to expel the fetus. So basically, she would know that something is wrong.
I will add a link of a photo where a woman was pregnant with twins, and one twin died because its blood supply continued to go to the fetus that survived, and even in this case, there was still quite a bit of fetal remains at delivery. WARNING! THIS PICTURE IS VERY GRAPHIC! It may be quite distressing to see, but it proves my point.
http://library.med.utah.edu/WebPath/PLACHTML/PLAC021.html

Now, if the woman was an alcoholic, drinking everyday to excess, then the alcohol comsumption can cause degeneration greater than what is in that picture, but again, unless she was not receiving any prenatal care, it would have been detected. Again, even with that, there would have still been some remains if she went to full term. And if she had no prenatal care, it still would have been obvious as there would have been no movement of the baby for quite some time.
So for whatever reason her mom is not being truthful, but I don't know that confronting her would do much good.
I do not know this woman, but I have heard women try to claim this to their husbands/boyfriends, friends and family, usually when they lied about being pregnant. This is actually a fairly common claim. In about 10% of those I have seen, the woman had some sort of underlying psycho/social problem.
Don't know how much this will help, but there you go. Susu