I had quadruple bypass surgery done about ten years ago at age 49. I was in good shape other than the clogged arteries. (Long history of this in my family) The sternum is completely severed and then wired back together. In my case it felt like a weight was on my chest. The only way I could sleep was on my back or very slightly to one side. Sleeping on my side was uncomfortable to say the least. I found that sleeping on the couch was a little more comfortable than the bed and that having my upper body slightly raised was best. It kept the sternum from feeling so heavy.
How is your dad's health otherwise? Is he able to get up from a prone position without help? The recliner may be more difficult to get out of than just getting up off the couch. Best case would be a situation where he can swing his legs over the edge and sit up with a gentle push from his arms. Not too much pushing with (or pulling) with the arms. You need about 5 to six weeks for the bone to reattach before putting to much stress on the chest area. The height of the couch should be such that his feet reach the floor with his thighs parallel to the floor. It will not take as much effort to stand that way and he can be seated on the couch easily as well. Having the upper body raised slightly will also help especially with a male since their center of gravity is higher than a womans. (Just bit of trivia)
I wish your dad a speedy and complete recovery. I feel great ten years on and no sign of heart problems.