My mum had a gastric band fitted about 4 years ago and had so many complications from it she nearly died.
While this was a different type of surgery than your friend is suggesting but I think a lot of the issues surrounding it are the same. It is not a quick fix or the easy way out, it is something which totally changes your life and not always for the better.
Your friend will never be able to eat a normal sized meal again, and there may very well be certain foods which she will never be able to eat again. I know some people might think this is a small price to pay for being ultimately healthier and having a potentially longer life but there are so many risks associated with this type of surgery. The bigger you are the more likely you are to die on the operating table or suffer serious complications.
I'm sure your friend knows all the risks and has considered it very carefully but after what happened to my mum I really want people to think so carefully about this surgery as problems can, and frequently do, occur. My mum lost about 9 stone - half her body weight, but the complications with her band caused her to not be able to keep anything down, not even water. Eventually she was so malnourished and dehydrated her organs started shutting down, by this point she had pretty much developed an eating disorder too.
Even though the band was killing her she begged the doctors not to take it out, and said that if they did she would keep making herself sick so that she did not put on weight. I know that if she had not had it removed she would not be here now, and I would much rather have my mum in my life!
Besides this. some people have deeper, emotional reasons for being overweight that you simply cannot 'fix' by taking away the physical problems. If you are addicted to food this does not change overnight, it can get replaced with another, potentially more destructive addiction.
It is very difficult, there is no easy answer but, unfortunately the reality is that diets don't work long term - whatever changes you make have to be maintainable for life, not ever eating chocolate or cake again is not realistic.
I am no expert on the matter I just hope your friend thinks really seriously about this, and if she does go ahead with it, she is one of the lucky ones as I would hate to see anyone else suffer for months like my mum did :(
By the way, if she has it done privately any complications or corrections that do arise with not be covered by the NHS!
Sorry for the mega post!