for how fertile she is on her period would depend on how many days into her period she was and how long her cycle is... without knowing a due date or if she already knows who the father is its hard to say for sure...
If she was pregnant she would already show positive on a test. Also, it is extremempy unlikely to become pregnant while on your period.
I would say if it was when her period was then your chances are very low. When she has her period she's not fertile. But I never say never. I wouldn't worry about it.