Quantitative hcG results

My 19 year old daughter just found out she was pregnant. Her worry is that she is not 100% sure of the father. She is however a good historian. On August 8, she had sex w/some guy she met at a party. He did use a condom. On August 15 she started what she describes as her normal period. Her period ended on the 21st. She then had unprotected sex on August 25. She then got worried (imagine that!) and between Sept 1 and Sept 6 took two home pregnancy tests which showed negative. On Sept 9 she went to the health dept where they did another pregnancy test, which at first showed negative, but shortly thereafter a very faint line appeared on the positive side. She decided that must be wrong so took another 4 days later, this time one of the lines showed positive, then by the time she took yet another test approx one week later both lines were visible. So yesterday she had a quantitative hcG test done at the hospital which showed a level of 63187.....I do realize that you really aren't supposed to read too much into the numbers, but yet it kind of startled me.....as guy #2 is a twin and my dad had twin siblings as did my mom. My feeling is that due to the "normal" period and that hcG isn't present until implantation that the father is the young man she had sex w/on the 25th.....just seems that since nothing showed positive til the 9th of September that that would correlate with implantation and the release of the pregnancy hormone.
Do I seem to be on the right track here?