Well...
It would have been nice if you could have gotten her email to discuss this. You could find out a little bit about what she wanted, what her expectations were, and what you're expectations are. Then there are phone numbers.
As far as characters, I'd probably make up 2 or three other npcs that I thought were interesting, and well made in the more than a supporting character sense. Then maybe let her and her boyfirend watch, as you split the party so she could see how things went and fold which ever character she chose into the story when the party regrouped. Without much backstory it's a little hard to guage how practical these suggestions are. But a small selection of decent level 6 PC/NPC's should take too long.
As far as how to fold a new person into group quickly a typical hunted plot seems like it might be the easiest. The enemy of my enemy is my friend and all that. No one has to trust of know or care about anyone but they sure might have incentive to work together.
That way she doesn't really have to bend to anyones preconceptions when she's likely to have some of her own...might just make it simpler and smoother. If you slow the game down a little so she can ask questions freely, so much the better. I used to do a thing where players would just interupt me, whomever, when they were going to initiate an action. If it took me off guard they might get surprise action, bonus to initiative, etc. I'm starting to drift, but I'd try to make it crystal clear up front, that if you're ever unclear, or need clearification interupt me to get it. It worked pretty well, even though it was a little harder on shy players. But if that would be something new to your current crop I think it would tend to even the footing somewhat, and if it isn't new, well I think it would be doubly important to spell out the protocol for her.
Oh and I'd be pissed about short notice for that sort of thing, I like a week so I can more completely procrastinate. My procrastination regimen is complex, even intricate, and cannot be rushed.