It depends where the disease is. It could be at the lower end, with disease lining the complete vessel. If there is nothing that the surgeons could do for the LAD, then I'm afraid it's down to pot luck. I say luck because it depends if collateral vessels open up to form natural bypasses, but this doesn't happen for everyone, hence most people dying during a heart attack. However, what concerns me more than anything is why he has such extensive disease? Has he smoked? because this with with high stress levels will certainly cause it. Is your friends cholesterol high? is the blood pressure high? what's the diet like? Whatever risk factors your friend has, they need to immediately be addressed and stopped to prevent worsening of the disease.
Sorry to hear that they were no able to bypass the LAD of your friend. If the LAD was fully blocked before surgery, other (collateral) vessel must have taken over the task from the LAD of supplying blood to the left part of the heart before the CABG. These vessels will continue to do that after CABG.