The problems I can see with her plan:
1. If she is too pregnant, the airline may not let her fly.
2. Does she know that she will have to pay the hospital bill for the baby to be born? I have heard around 5-8000 dollars for that.
I don't really know if it would be a problem with the visitor visa, they don't ask on it if she is pregnant. When she gets there, if she doesn't look pregnant, they will not ask her but if they ask her I have no idea what will happen.