Bariatric surgery requires a change in diet before as well as after the surgery.  Protein shakes often become an integral part of that diet with Whey Protein Concentrate, such as Eat Me NZ 100% Whey Protein being a suitable source of protein prior to the surgery and Whey Protein Isolate, such as Eat Me Lean Shake  after the surgery. 
L-Carnitine may help increase metabolism and improve mood during the pre-surgery weight loss phase. Glutamine may assist with weight loss and post surgical healing. Please discuss these options with your nutritionist first.