Get with his surgeon for suggestions. Don't assume that he's all that fragile. A lot of times people are over-restricted after heart surgery. It's my profession to deal with this sort of thing; I'm an occupational therapist working in an inpatient rehab setting. People who have had heart surgery usually do just fine with any activity that doesn't cause pain. With that said, how long has it been since his surgery? He should have plenty of time to heal the incision, and he should have his doctor's OK before he hits the range.
Once you get all of that covered, the above suggestions are good ones. My father-in-law shot his Garand with a lead weight in its stock after multiple open heart procedures. Breaks will reduce felt recoil, but add blast and may be too harsh in that way, but adding weight to the stock and using a lighter buffer are both solid and non-obnoxious approaches.