Unfortunately the calculations of the due date by most in the medical profession is not statistically accurate and have not been adjusted based on evidence for decades. Race, age, weight and habbits all play a roll in it. Let me know if you want a link to an accurate calculator based on the latest research and statistics.
I think it is impossible to say who the father is based on the dates. You had sex with two different men during your likely fertile period. The window for conception can be as long as 6 days.
However, if you had protected sex with your friend as you are stating, he should not be the father - that is after all why we have protected sex.
Hope it works out for you.