Like Tundralag said, it might require a 2nd trip, but it shouldn’t hold things up with your deal.
A few years ago, I bought a Tacoma and was trading in another vehicle for it. My wife was not with me when I bought it, but her name was on the registration. They had no problems closing the deal (even through financing) and then having her come back a couple days later to sign the forms.
Now, if it’s a dealer that is far away and you do not want to go back to, then I’d recommend getting a power of attorney that grants you full rights to sell the vehicle on her behalf. We do it all the time in the military because when we move, we need others to help ship or sell our vehicle after we have left. Just make sure it has the vehicle details such as VIN, license plate, mileage, etc in the POA...and make sure it is notarized properly.