I can understand her not wanting anyone to know. I told about 5 people. 3 of those people have been FANTASTIC. Asking questions of concern and interest, and being all-around encouraging. Another friend just doesn't get it. And the last friend called me "lazy". So, not everyone will have the same reaction. A lot of people do not know what it is like to be overweight their whole life, and think just going to the gym will automatically "fix" it.
Also, this procedure is probably a lot safer than her being overweight for the rest of her life, or her constant "roller coaster" dieting. The risk of death is super low.
One thing you might want to avoid is being the "food police", that is, reminding her of what she can and cannot eat. My mother likes to constantly remind me of the food I won't be able to eat again, and that I'll never be going out to eat at restaurants again, which isn't necessarily true. Most bandsters can eat most food again, just in smaller quantities.
The hardest for her will probably be right before and right after surgery, mostly the pre-op and post-op diets. The diets restrict what she can and cannot eat so she can shrink her liver for surgery, then allow her stomach to heal.
I think it's great that you are seeking help from other people as to how to help her out. You are a great husband!