Actually after thinking more about this, I would ask the cardiologist to be more specific about his using the term 'occluded'. If the veins are blocked, it could be that the veins have 'collapsed' which is not uncommon, hence the reasons they use arteries more now.
This is an interesting exchange of views between doctors. Veins dont suffer atherosclerosis but can suffer thrombosis. When used as arteries in a bypass, I assume they can develop atherosclerosis and block up. This makes them no different to arteries in that respect and so I can't see there being a different outcome from using either veins or arteries.
In bypass surgery it has become much more common to use the mammary arteries which lie in the chest and are already attached to the aorta. Arteries are much stronger than veins and last longer in bypass situations. However, mammary arteries are checked during surgery to see if they are healthy enough to be used (i.e. have a good flow). If they have hardly any flow, then there is no benefit in using them on a patient for a bypass. The surgery notes would have to be accessed to see if this was the case with your Father. The surgeon usually makes his decision during the surgery and his decisions are noted down for records. Has your Father lowered his cholesterol to the required levels in a patient with CAD?