Hi,
I have had a couple of surgeries since being diagnosed and have had no issues whatsoever as far as increased weakness goes. However, bariatric surgery being somewhat "elective" and one that will affect digestion and metabolism forever is one I'd try to forego if at all possible. Trying to lose weight "the old fashioned" way would take longer but is without the risks of surgery and the aftermath.
If many serious, medically supervised weight loss attempts have been made already, and surgery seems the ONLY solution, then I would definitely get a neuro on board before the surgery, and to help with any needs after. Any surgery has risks in and of itself, and MG only increases the need to be cautious.
Good luck to your wife!