Lent et a (2013) showed that whether or not some one was a smoker did not affect weight loss, even when smoking rates did not change.
Lent, M. R., Hayes, S. M., Wood, G. C., Napolitano, M. A., Argyropoulos, G., Gerhard, G. S., ... & Still, C. D. (2013). Smoking and alcohol use in gastric bypass patients. Eating behaviors, 14(4), 460-463