Impossible. Early pregnancy tests can detect rising hCG levels about 5 days before your missed period, NOT after conception (fertilization).
A fertilized egg does not implant until 7-10 days after fertilization. hCG levels do not start rising until implantation. Detecting pregnancy 5 days after fertilization is impossible.
It is very unlikely she will get pregnant from this, given the timing of events. But here is a clue - if you are having sex, then be smart about it. There are so many advances in birth control - there is no reason anyone should be going through this. Unless they are stupid and irresponsible.