You don't say what your plans are...are you riding up, hiking, biking? Will you be just going to the top and then down again, or are you planning on hitting all of the stops? Is he weak, feeble, doddering? Or is he an active man. Like FL said, let HIM set his limitations and enjoy the trip with him.
I have CHF...I recently took a trip out to the southwest. I was not overly bothered by the altitude, but I think my highest altitude was maybe 10-11 thousand feet...also, it was a road trip, so I think I adjusted a bit simply with the travel.
What does his cardio say.
I have seen people affected by the altitude. Now mind you, I know mostly flatlanders. Your Dad is 78, if it is his desire to go to Pikes Peak I certainly think he will do so. I am not at that age yet, but if my kids tried to stop me from doing what I want they would certainly wish they had thought twice. Unless you Dad is mentally incapable, let HIM set his limitations. Enjoy the trip with him. Let him LIVE and experience and not just watch. My mom is 82 with stents and bypass, she is always on the go. My father in law is 78 like your father, several MIs and Quadbypass, I cant tell you how many times I have seen him climb to the third storey. It would be a lot easier to watch if he did it inside on stairs instead of OUTSIDE on the steep shingles!!!