To my knowledge general anesthesia is no riskier for an SLE patient than the general population. It has it's risks, but they are minimal (to miniscule!) with today's excellent anesthetics.
SLE patients can have longer recoveries from surgery itself, or higher risk of infection or blood clots. This would be possible for those on immunusuppressant medications or high dose prednisone with respect to infection, and the blood clots may be relevant for those with anti-phospholipid antibodies and prior clotting problems. She will want to discuss these issues with her surgeon or have her rheumy consult with the surgeon.
All that said, most lupus patients do just fine with surgeries and recovery from them. Certainly has been the case with me.
Best wishes to your mom for a successful surgery and a quick and uncomplicated recovery!