Well, as you may know, early ultrasounds are the only ones that can be used with any certainty for dating a pregnancy. 15 weeks pregnant is too late in the pregnancy to date the pregnancy from an ultrasound with precision. By now the margin for error is +/- 8 days or more.
So that makes the big question did she get her first due date, the one that was given to her as August 27, from an ultrasound? If so, at what point in pregnancy was that ultrasound? (If she doesn't know off the top of her head, ask her to look up the record.) If the due date of August 27 came from just counting on a calendar or a little cardboard wheel, it won't help, but if it came from a nice, early ultrasound, it is the due date to rely on. Not the one from the 15th-week ultrasound.
You said you were having unprotected sex as late as December 8? If so, on the morning of December 7, she would have had live sperm from both of you in her system. The August 27 date (if it came from an early ultrasound) suggests a conception window that includes both of you and slightly favors you, November 30 to December 8. The September 1 date also suggests a conception window that includes both of you, September 5 through 13th, and doesn't favor either over the other. Remember that sperm can live up to five days or more in a woman's system.
I think there are a couple of things you can do and one to consider. First, find out how she got the due date of August 27, and when in the pregnancy she got it. If she got it in her 6th or 7th week and got it by an ultrasound, not just a doctor's estimate based on her last period, then you can count on it as the most accurate. Second, go with her to the doctor appointments if you would like. I am sure that if a DNA test after the baby is born proves you are not the father, you will be able to detach emotionally, though you and she might always remain friends because you helped her during a difficult time. Third, you and the other guy could split the (very hefty) cost of doing prenatal DNA testing. It is not perfect (we heard one story on this site of it not working at even the best lab, but at least when it was proven wrong the lab did refund the money) and it's very expensive, but from all other stories it sounds like the two good labs are reliable, so you would [almost undoubtedly] know quickly who the dad is. If you want more details on the labs with the good names, here's a link.
http://www.nytimes.com/2012/06/20/health/paternity-blood-tests-that-work-early-in-a-pregnancy.html?pagewanted=all&_r=1&
Well when they said she was due on Sept 1 she was actually 12 weeks 4 days that was with ultasound that was on Feb 21st 2014 and as for as the Aug 27th date that was not an ultrasound at all just going by the last PERIOD.... I think she was like 6 weeks when she Told her about that and I was having unprotected sex with her for a while all last year and my birthday weekend Dec 1-4 we did our thing and on the 6th or 7th she did her thing with the other guy and the condom BROKE...and I kno I'm just afraid to get all emotionally into it and stuff my friend wants it to be mine and everybody else we known each other for 6 years so it's just a toss up I guess
Since you only have a date from a 15th-week ultrasound to go on, yes, it's a tossup. You and the other guy have pretty much even chances of being the dad. See the second and third pieces of advice above, since the first one can't apply. Sorry it is not going to be clearer for you until you do a DNA test.
Well she just told me on January 1 they told her she was 6 weeks and in the 18th of January the doctor gave her the due date of the utlatsound that she saw on the 1st.... And of course the other ultrasound she had on the 21st of Feb the tech said she was 12 weeks 4 days....DNA testing inside the womb is really expensive money I don't have....and it's ok it is what it is life is a B thank you for the advice I'm not going to leave her high in dry I'm just not that guy I'm just going to have to hope for the best and prepare for the worst
The prenatal DNA testing I was referring to is not the kind that could be described as "DNA testing inside the womb." It is done by blood test (drawn from the arm). It is definitely out of reach for most people because of the high cost, though.
Good luck, time will reveal what has happened. I wish there were a faster answer than waiting 25 more weeks, but as you say it is what it is.