Have Gangrene In Both Legs. Was Hospitalized For Alcohol Induced Dementia. What Happens When Gangrene Spreads?

Last September 2012, my brother, age 59, was hospitalized for alcohol-induced dementia, along with gangrene in both legs. His right leg was amputated above the knee in early December and because he wasn't eating, a feeding tube was inserted in early January 2013. At the same time, gangrene was evident in his left leg. At that time the plan was to amputate when gangrene spread to above his knee. The gangrene is now about his left knee but he has been declared inoperable due to his fragile condition. His toes are so fragile that an abrupt movement might cause them to fall off. It is inevitable that the gangrene will spread. My question is: what happens when the gangrene spreads? Does it get into his organs and cause death? (We know he will die of this; it's only a matter of time). Thanks.