Believe it or not, this isn't all that uncommon. Someone in my workplace is going through this very thing. She lost a tremendous amount of weight after gastric bypass surgery and even had the abdominoplasty to remove the excess skin. Now, she's slowly regaining everything she lost.
If you really think about it, WLS forces the patient to eat very little for a prolonged period of time (several months at least). If you don't think that will seriously wreck your metabolism, think again. I'm honestly surprised that as many manage to keep the weight off as they seem to claim.