I wouldn't put your cards on the table, you might upset your friend and make her question her own relationship with you.
I'd make an excuse, don't go in to too much detail, just say you've double booked or can no longer make it because of X Y or Z.
I used to worry and be a people pleaser but, whilst it goes against the grain somewhat, I am learning to say no.
I don't my like when people just spring it on you that someone else is coming, one of my friends used to do this a lot, invite you for coffee then you get there and oh I've asked so and so too, or we'd go for lunch and she'd bump into a friend and without asking me if it was ok first, all of a sudden friend whom I've never met is being asked along to lunch with us (I sound like a right arse saying that, but just to be asked would be nice, it's not something I'd have done the other way round either)
I used to meet up with a good friend and one of her friends, whom she'd known longer than me, one time her friend was early to where we were having lunch, we entered the restaurant sharing an umbrella and her face looked like a slapped arse, she didn't shake it off for s good half hour and then it was the usual taking a trip down memory lane and what great times they'd had and she couldn't believe they'd be friends so long, it didn't bother me per se but it was irritating and try as I might I just couldn't gel with her, so after a handful of social occasions, any further invites were met with an "oh, I'd love to but I can't manage" I think my friend soon read the situation.
I am learning that you're never too old to be let down, or to feel disillusioned with friendships at times.
If DC doesn't know hen she won't be missing out, go with your gut, and do what YOU want to do, not what you think will please others.