Depends on how strong a relationship you have with this friend and what her personality is like. There are diplomatic ways to talk with her about this. If this were me with a friend I would be able to say, "I talked with my husband and he's upset because the Blue Book says the value of the car is more like $3500. My good friends would say, "OK, I'm disappointed but I understand." They would then say, let me think about whether or not we can afford $3500. They might come back with an offer somewhere between the 2. So might I.
Or more likely, we'd put the car on the market and if it didn't sell for $3500 give the friend first option on making a bid. I would give her the inside track.
Because she's a friend, I think it's important that you're honest with her. I also think it's important to get more money for the car if you can. If not selling her the car for $2000 ruins your friendship then the friendship was not strong enough to withstand what is really a minor glitch in communication. She should understand that you were talking off the top of your head and without thinking. If she didn't think of it at the time, she should've later realized that you cannot sell the car on the spot without your husband's input. If I were her, I would also have gone home and looked up the value and gone back to you and said, "are you sure about that price?"
If you don't talk with her you will have feelings that will interfere with your relationshiip. Best of friends stay best of friends because they are honest with each other.