I haven't had experience with this exact situation, but my dad had a minor heart attack
about 7 years ago, and they put stents in his arteries and he has been fine since then.
As for the surgery, there's a risk of dying anytime you have surgery done, because of the anesthesia
(stuff to put you to sleep). But, the doctor must feel that the risk is small or they wouldn't be doing it. My grandma has had surgery many times over her life, and always came out of it just fine. But now, because of her age and the fact that she heals slow (most elderly people do) they don't do surgery on her unless that's the only thing they can do.
Your dad should be okay; depending on what kind of surgery he's having done, he should feel better within a few days to a few weeks. I know you said it was his heart, but you don't say exactly what they're doing- if it's just stents, he'll probably be out of the hospital within a couple of days and be back to his normal routine within about 3 to 6 weeks. If it's open heart surgery- like a bypass- they'll keep him in the hospital longer and he'll need to take it easy longer once he gets home. But, as long as he follows the doctors' instructions he should be fine.
The best thing you can do is try to relax, spend time with him (maybe bring him some flowers or a stuffed animal) and pray. You have my prayers as well.