You have to be a BOSS!!
lol jk
If you don't know the girl too well then this can be hard.
Build a connection with the girl and then ask her to a game. Take her to the game and have them write on the monitor "Will You Go To Prom With Me?"
That would prolly make her want to go =)
Or you can try more traditional ways...
Probably the easiest thing to do is to just ask her. But I'm assuming they are going as friends, right? In that case it shouldn't be too hard because there's no pressure. If she says yes, great! If no, then your friend has time to find another prom date...
HOWEVER...
If your friend is asking her because he is interested in her, then that's a whole other thing. The best bet in this case would be to get to know her first and see if she is interested in your friend. If she isn't or already attached to someone, then your friend is kind of out of luck. But since it's so early in the school year, your friend has the whole year to get to know her, and vice versa.
Nothing wrong with asking early and writing the question on the hockey stick.
My friend wants to know how he asks this girl to prom... He has brought her a mini hockey stink and he is thinking of writing something on there but has no idea what to write he doesn't want to write the classic " will you go prom ...." Has any of u got and good ideas of what to write by the way she loves hockey