pregnancy

A male friend of mine is currently in a bind. He had sex with this woman only once and she called a few days after stating that she was pregnant with his child. I know that the true paternity can not be establish until the child is born; however, the story just doesn't add up. They only had sex on Nov. 16, 2010, in which he stated he wore a condom. On Nov. 21, 2010 she called stating that she took three pregnancy tests stating that all were positive. She took a blood test at her doctor's office on Nov. 24, 2010, which confirm her being pregnant. On Dec. 3, 2010, she had an ultrasound which estimated her due date at July 17, 2011.She stated that the doctor informed her that she was 7 wks, 5 days at the time. She stated that her LMP was Oct. 9, 2010 and this is how they determined the 7 wks, 5 days. Isn't there other requirements that determine the age of the pregnancy? Is there anyway he could be the father of the child? She is convienced that this child is his? Is there a clear destinction between between a 4 week and a 8 week ultrasound? Would the doctor necessarily go to with the LPM over the ultrasound measurements.