Thanks for query,
Your grand fathers coroneries were blocked by about 90% & he needs bypass surgery
I think along with lungs his kidney
status should also be checked before deciding for by pass surgery.
If he had good functioning kidney's the chances of by pass result is increased.
to go for surgery or not is an individual decision as at this age there were much more risk on operation table for anesthesia
In a sedentary life style medical management can also serve your purpose. thanks.