I agree with vienna that age is a factor to consider and treat accordingly, but with age aside the completely occluded artery has collateral vessel development (natural by-pass) and not warrant surgery. A stent may not be physically possble due to location and/or angle of lesion and degree blockage. I have a completely blocked LAD with collateral vessels, dx'd 4 years ago and also valve leakages. Medication serves me well.
The proper medication regimen is to dilate vessels that reduces the heart''s workload can/will decrease shortness of breath, and a nitrate that quickly (or slow acting) and specifically targets the coronary arteries can/will dilate those vessel and relieve chest pain (angina).
She is 90 years old. In my understanding 90 is old for that operation. but I am not a doctor. Give her love, good companion and good diet. Also you should ask that question on the expert Forum. My best.