If you want to make it REALLY rules - light on her, write up her character sheet for her.
Since you have time, ask her what she wants. Otherwise, just make a selections. Before the game, tell her to pick the character whose personality seems interesting, that she'd like to play as.
So, you could whip up, say, Human Fighter or Half-Orc Cleric or whatever you think the party could use and she would enjoy. Then create a few different personalities for her to try and play. Emphasize that she can change the personality if she'd like, etc.
Either way, when game day comes, you can guide her through it as she's doing it. You can say "Make a melee attack roll. That's going to be one twenty-sided dice rolled, plus the number in the box that says "total melee attack bonus" in the corner. If the roll of the dice, plus that number, beats their armor, you hit and can roll damage."
Deal with things on a case-by-case basis. Emphasize that it's not important to know what everything means just yet.
Heck, I did that to introduce three new people, all women, to gaming. Now they enjoy it a lot, and me and my friend are the only guys in the group.
It's good to be outnumbered. ^_^
But, to recap:
*Write out her character sheet. Giver her a choice of characters to choose from.
*During the game, tell her where on her character sheet to find things.
*Remind her that it can all be changed, and that the game's point is to have fun, and some people just don't have fun playing it, and that's okay. Even though you think she'll have a blast (no sense in letting a potential dislike of D&D get in the way of a friendship, eh?

)