OP - what was your period like in August? Was it light? Reason I ask is you said you tested on 15th of September and "it was positive @ 2/3 weeks". Those tests don't actually mean you're 2-3 weeks pregnant, they mean your HCG levels are high enough to indicate you implanted 2-3 weeks ago (august 20).
In order for your friend to be the father, you would have had to ovulate on the exact day you had sex with your friend (egg only lives for 12-24 hours). You also needed that fertilised egg to travel down your fallopian tubes and implant into your uterus (a journey that typically takes about 7-10 days) and then produce enough HCG to show 2-3 weeks pregnant SEVEN days after having sex with your friend.
Most of the time the egg hasn't even implanted in the uterine wall by then and most people can't tell they're pregnant until a minimum of 10 days after ovulation.
I truly believe that you implanted or ovulated on August 20th (and it wasn't a period) and therefore there's no way its your friends kid. The dates just don't make any sense for your friend... especially since you also said:
I had a scan 8/1/2015 and that dated me at 20w and 2days?? I also had a scan after bleeding on 11/12/2014 and that date me 16w and 2 days??
Your dating scans are based on growth of the baby (rather than the date of your last period)
So the 8/1 for 20w 2days = August 18
The 11/12 one of 16w 2days = August 19
I'm 99% sure your DP is the father based on the dates from your doctor.