I'm having a flashback to my dad, Emmy, and I know how scared you are, but *the problem was caught while there's still time to fix it*. I'm sure your family has all the paper to read they can stand on the subject, but the double bypass means that his heart was healthier than most who go through bypass procedures. That's working for him, his relatively-young age is a positive factor - with lifestyle and/or dietary changes after the surgery and medication treatment, you'll have him around for a long time. We recently lost a cousin who had had quadruple bypass surgery 28 years ago and lived to be 91! Surgery, treatment & medication have all made vast improvements since then.
Add my prayers as well. Let us know.