...the series would have no interest to me after that.
And Bella isn't even a party person- but i guess this is your paralle universe so let's see....
Bella would freak out. She'd probably be pissed at said boy and storm out the room crying.
She'd be embarrassed.
Live in denial.
Edward would probably would find out somehow cuz of Alice or something, and feel sorry for her and maybe try to help Bella.
Bella would go into hysterics and ask Edward to go to bed with her...
Edward might give in, but probably won't because he would technically take advantage of her vulnerably....He might even want to kill the boy...(haha I'm on a roll)
Bella would soon find out she's pregnant, find the boy and i guess somehow fall in love with the him? I don't know how. Oh, the boy could be Jacob's cousin. He'd be similar to Jacob but normal.
wink She falls in love with him cause she won't have to worry about being killed by angry vampires and not worrying about Jacob trying to seduce her.
They get married causing both Edward and Jacob pain and they both work together to take out Bella's baby's daddy. ^_^
I took a stab in the dark with that one.