She said she didn't have a period last month either. So does that look like its not my friend's? Since I think her periods are around the start of the month.
Look, she might be mixed up on her dates anyway(or she might want your friend out of the picture), there's just no way to tell at this moment 100%. We'd all be guessing, & relying on whatever she's told you, which is probably true.
I have been told by enough women that their cycles screw up occassionaly, so missing a period isn't 100% a reliable indicator.
Hopefully, a doctor could guess from the development of the fetus a good estimate of pregnancy stage, but then again, they rely a bit on the dates when the woman has told them that they've had sex.
She could have an amniocentisis at 18 weeks ( she might have to anyway if she's 35+), at which point paternity could be established. Other than that, you'd only know via a dna after birth.
I had a similar situation - I had sex with a girl, & wanted to keep on seeing her(what she could do with her body-wow), then 2 weeks later, I was at the doctor's with her. She reckoned, & the doctor too,that she was 4 weeks gone (that was him basing it on what she'd told him, & her usual cycle), & it was her on off long term boyfriend's. I broke up with her,but not because of this - I just didn't have anything in common with her.
It turned out ultimately, that she'd slept with another guy, the night before me, & after a rigarmarole of dna testing - she had only been 2 weeks pregnant, & it was his.
So, really, your friend is never going to know 100% until the child is dna tested.