Having had bypass surgery myself, I'd say the probability would be that Mr. Kelly will be back at work sooner rather than later. I was doing a limited amount of work (media sales) from home less than a week after my operation. People I spoke with on the phone said I sounded perfectly normal. As if nothing at all had happened. So the part of it for a person making a living with his voice is pretty easy.
The main issue during recovery isn't the heart. That's now been repaired. Instead, the main issue is recovering from the trauma of the surgery. Lots of tissue in the chest needs to recover and reconnect. Even when you start to feel more like yourself, you need to be on guard to avoid any strain which could delay or cause setbacks in the healing process. This was the limiting factor for me. It didn't stop my from gradually ramping up my activities on the phone or computer from home. It's also worth noting that for a few weeks, you're not allowed to drive. Not that you couldn't. But if you were involved in an accident....or even a sudden stop...you could seriously disrupt the healing of your chest (or worse).
So I would say if Mr. Kelly has a studio in his home, he could probably be back at work within 2-3 weeks of the surgery. Perhaps another 2-3 if he would need to drive himself to a studio. In my case, I had surgery at the end of a September. By the end of that October, I was driving to the office 3 days a week. By mid-November I was a daily commuter. And by the short week before Thanksgiving, I was on my first out-of-town overnight business trip...including airline flight and rental car.