The LIMA is blocked. LIMA (left internal mammerian artery ) was used for the orginal graft. It is convenient to harvest, seldom present complications and it has excellent long-term benefit. Unforutnately, a peripheral vessel harvest may be the only opttion and isn't as good due to possible wide spread artherosclerosis (hardening) and stenosis (narrowing). That would be the main concrern as there are a limited supply of grafts available.
I know a few people that have had to have a second by pass at the age of your father. Brother-in-law has had three and was told there were no other grafts available. A doctor would know whether current physical condition is adequate to have another surgical operation, and age wouldn't be a factor to not have another by-pass.
Your father is receiving good blood flow from collateral vessels. Collateral vessels are nature's way of providing a growth of vessels to by-pass occlusions and supply blood to areas that are deficit of good supply (anatomosis)..