Hi Beth!
You sound like such a fantastic friend, your BFF is so blessed to have you on this journey with her.
As far as how to support her, that varies so wildly from person to person that it's hard to say. Definitely the help after surgery will be wonderful for her. Those first couple of weeks can be a tough adjustment.
Pre-op, I would encourage her to go to all of the support group meetings and therapy sessions that she can. As my surgeon said, "We remove your stomach, it's not a head transplant." Spending some time in therapy to deal with my food issues (and the root of those issues) has helped tremendously.
I understand her fear of having to change the WOE for life. It's hard to get our heads wrapped around, but it's so very important. That's why most plans have 3-6 months of pre-op dietary counseling. It's hard for her to imagine now, but she likely won't be hungry for a long time after surgery. I get hungry (for some reason that never went away for me), but it's not like it was before. And obviously difficult to explain, lol! Also, again hard to imagine, but such a small amount of food truly satisfies now. And the consequences of overeating are. NOT. worth. it. Now, I eat 1/2 a chicken breast and I literally feel like I just finished thanksgiving dinner I'm so full. There's a learning curve on how much you can eat, but it gets figured out fairly quickly. If I eat even ONE bite too many it makes me terribly sick and in excruciating pain. Again, simply not worth it.
Good luck to your friend, please ask any questions you need to here! We're a friendly bunch.