Can you ask her to do a test? Tell her to poas so you can check it out. Or take her to get blood work. Maybe you can lie to her and tell her that she needs to get a u/s done, or blood work done to confirm a pregnancy. And then there she will see it is not true. That is a tough one, if you think that she is harming herself with this (and it sounds like she may be emotionally) then I would come out and tell her the truth.
I have heard that women can want to be pregnant so bad that their bodies can actually follow the traits of it. Thier mind takes over and believes they are pg and their tummy will grow, they will feel things, I even heard you can produce milk!! Crazy eh? That's too bad, poor girl needs a little help, it is going to be so hard for her when there is no baby in a few months.