I don't think I would throw the party. But that's just me.
They had a big fight last weekend and he left but then came back the following day. It doesn't sound like she can count on his best friend to participate if he won't even return her calls or e-mails. At the very least, the surprise element is gone whether or not his friend told him about the party. It seems to be the guy would at least say it's not a surprise anymore and the cat's out of the bag.
I guess she should talk to her boyfriend and tell him that she's wanting to plan a party and was hoping it could be a surprise, but that doesn't seem to be working out and see what the boyfriend says. If boyfriend says, "My friends already have a party set up for me...." that would explain why his bff isn't getting back to her about helping with her plans.
She can tell her boyfriend "So and so was supposed to help me make this a surprise, but he's not speaking to me, so I guess the surprise is out of the question."
If she gets any himming or hawing about the party, she should call it off and make plans for her girlfriends instead.
Boyfriend might have gone to his bff and griped about her after the fight and bff doesn't want in the middle of it.
She can tell her boyfriend what her plans were and if he blows her off.....she knows no party is necessry.
That's just my opinion. I know she wants to do something nice for him, but maybe now isn't the right time.