Could he be the dad?

My brother has gotten himself in a tough situation. He had sex with a girl last Tuesday and it was protected. She came over yesterday with a positive test. She also took another one in front of me. The line was dark y'all and popped up immediately. Is it possible that the baby is his? He's over here stressing out not wanting to go and tell our parents about the possibility. I know it takes a couple of days before fertilization and implantation but I don't really know how long. Please help!