You don't agree to the date. Nicely turn her down and suggest another time when it can be just the two of you. Period. End of story. Sort of like, "Hey, thanks, but I'd rather do something just me and you. How's next Sunday? Let's go see it then."
The reason you don't want someone else there (not even if you find someone for that person or double date) is that it's a distraction between you and the girl and interrupt whatever vibe you're creating, maybe even cut your date short because they don't "feel well" or whatever.
Also, the other person can act like a c@ckblocker if they don't want to "give up" their friend to you.
Also, now you have to "entertain" their friend(s), IOW, they have to like you and as they're not there because they're interested in you in the first place, that can work against you. And if they don't like you, they'll influence her against you too, especially if this is one of your first dates in with her. So, why take the chance?
You have to date smart and reduce the odds of things happening that could work against you by running a tight ship.
squirrels said:
A movie is a crummy first date.
True. It's passive and you're just sitting mutually staring at a screen, rather then having fun interacting together. That's why scary movies are better. But the reason he should go to the movies in this case is because SHE invited him. That signals some interest and it should be rewarded. It's just that she wants to bring along someone and that's not acceptable.