In carefully selected people, a heart transplant can be very successful.
About 79% of all people who receive heart transplants survive for at least 1 year, while 75% survive 5 years and 60% survive 10 years. Recognize these statistics address a majority of older recipients who often have other maladies to complicate the factors. Statistics for younger patients is much better.
So your friend has a better than even chance he will survive as long as you do. And remember, our medical skills, products and experience improve significantly every year.
Yes, his experience is a serious matter...but in the end, we all face a certain death and to varying degrees to the what and when of our demise. I suggest that you and your friend concentrate on what is most important - to live each day as fully and happily as you can...because any of us can get run over by a truck tomorrow, and there is no practical advantage to worrying about it. In fact it is a lousy investment to dwell on what might be. What is, is ... what will be, will be...except what we do within these time frames decides the value of our lives.
So give him a big hug, and get on with life!