If sperm was ejaculated on her back and wiped off with tissue then she took a shower. could the water drain off to genital area and cause pregnancy?

Very unlikely: The issue would be if you didn't use a condom during actual intercourse. Most men produce small amounts of precum during sex. Each drop contains about 20 million sperm. This is why so many couples get pregnant with withdrawal. The semen on her back however, very unlikely to cause pregnancy based on how you wiped it off. Dry air kills sperm