Treatment for Blockages at Bypass Sites

My dad had bypass surgery 4 1/2 years ago.  He has been found to have 100% blockages in two of the three bypasses.  He is being treated with Coumadin and surgery is not even a consideration.  Why?  I have not been able to speak directly with his cardiologist and my parents tend to get things kind of mixed up.  Is surgery not an option in past bypass sites?

[Cleveland Clinic]

Re-bypassing someone is an option for patients with significant recurrence of symptoms, but it depends on which bypass grafts are open and which are closed. If he had a IMA graft placed, this graft lies in front of the heart and is susceptible to damage during sternotomy. For this reason, and also because this graft has the longest life expectancy of any type of bypass, re-CABG is usually only offered if this graft has occluded. The other grafts may have occluded because occlusions of the arteries the bypass grafts supplied were not very significant.

[horst]

A related discussion, bypass surgury was started.