Well, it could go either way. His heart, by itself, is strong and healthy with no damage at all. (Years of cardio exercise through drumming...

)
It's two of the four arteries going into the heart. Three of the surgeries were to repair those arteries, but to no avail. They have closed completely. If he can keep his diet and lifestyle fairly strict, the heart can continue to build those new arteries.
You just never know what overexerts it. One immediately thinks of exercise and extreme physical activity, but mental activity and stress can factor in as well. He can be sitting at his desk at work, mentally working away at a project, draw a deep breath and BOOM! Massive chest pain because the heart is overexerted from trying to draw a deep breath. Or just standing up too quickly. Or nothing for a couple a days even with going to cardio rehab three days a week. There's just no predictability because of the re-building process.
Doctors have only discovered diastolic heart failure recently. It has given them more details into heart surgeries and what can be done and what can't be done, but it is still relatively new. Yay, Billy for being part of something new!

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"Who among the gods is like you, O LORD ?
Who is like you—
majestic in holiness,
awesome in glory,
working wonders?" Exodus 15:11
http://www.achurchforthecity.org"I don't reject your Christ, I love your Christ. It's just that so many of you Christians are so unlike your Christ." Mahatma Ghandi