If you are used to Vegas vacations, perhaps your friend would like to spend some time in DTD shopping? Or perhaps at the Grove, which is still opening stores and restaurants. I know that doesn't solve the
Disneyland dillema for you, but it might be something your friend would enjoy, and this is everyone's vacation. Then maybe she would be more willing to go along with your Disney plans.
The one key thing with planning a trip with non-planners is to pick one 'land' and do all or most of the attractions there. I know that sounds obvious to Disney people, but her family might want to run from Space Mountain to Indy, to the Matterhorn, to Big Thunder and then to Splash. That's a lot of criss-crossing the park, taking up time and making everyone extra tired. Try to guide them to the other attractions in that land, since you're passing right by them anyway.
They have 2 girls, 11 and 6 1/2. The younger one is very small stature so she may not be able to do many of the bigger rides, while her older sister loves the big rides. It may work to split up and say we will take the older one with us to do one or two of the bigger rides while you do something with the younger one.
How close in age are these girls to your children? I think the younger girl might feel quite left out if she's not able to go with 'the girls' especially her big sister. Would it bother you to ride some smaller rides or is it all thrill rides for your family? Can you teach her parents about 'kid swap' for things she doesn't want to or can't ride? It might slow you down a little more, but it would be a kindness to share some Disney secrets like that with your friends.
How long are you staying? Is there enough time to show your friends the 'lay of the land' and then let them on their own for a bit? I would suggest to them to take a Walk in Walts Footsteps tour, or some other tour that gives them some Disney background. But then, I love tours (and I go to Disneyland all the time!)
Try to relax and slow down. There are lots of people involved in this trip, I'm sure there will be lots of compromises from everyone.
Enjoy the magic!