here's some basic information to get you started - http://win.niddk.nih.gov/publications/gastric.htm
are you able to talk with your mother's surgeon about having the surgery? most surgeons require people to attend a general information meeting before making an appt. It's important to understand what you're getting into - the good, the bad, and the ugly.
and the sleeve [aka vertical sleeve gastrectomy - to give you another search term] is indeed irreversible - most of your stomach is removed.
REMEMBER: surgery is NOT a quick fix - it's a FOREVER commitment, and most of the battle takes place in your head.