How is your father feeling right now?
If, with proper medication, your father can be made free of symptoms, you could perhaps try to go for medication alone.
If symptoms do not go away however, your father remains in a dangerous situation where a heart attack can easily happen.
Medication alone will not solve the stenosis. 80-90% stenosis is significant and can easily evolve to a complete blockage, which would mean that your father would suffer a heart attack.
I can understand that the cost of PCI is a problem. Still, I think you should take the recommendation very seriously.