She should get the ultrasound soon because they get less accurate as time goes by for trying to assess when
conception was.
At this early stage, the doc should be able to judge when
conception was to within a couple of days.
If the ultrasound suggests conception on 7/30 or even 7/31, you might still be the dad since sperm can live so long in a woman's body. It would be helpful if the ultrasound instead suggested a date like conception on 7/27.