SM, sorry to hear of your terrible news. I hope that my colleagues posts go some-way to answering your questions.
You ask whether or not this type of problem could be detected. Well in most cases (but not all), the patients present with some warning symptoms, for example pains in the chest ("angina pectoris"), which lead us to investigate further, (for example angiography, a technique where the coronary arteries are illuminated, demonstratinig area's of blockage.)
In other cases we manage risk factors for patients. Lowering bp, stopping smoking and changing diet, are particually useful for those who are at high risk of developing heart disease, for example those with a strong family history.
In other patients, unfortunately we don't know there is a problem until it actually happens, and no amount of examination, or routine investigation, can predict a problem until it occurs.
Without knowing the full facts surrounding your friend's death, I guess that he may have fallen into this, last category, from the details you have posted. Hopefully the technology will develop further, so that, in the future, we can identify people like Paul, before a problem occurs. Unfortunately, at the moment, we don't have this technology.
You ask is it worth you continuing a healthy lifestyle. Well definately yes, as you will be loading the odds in your favour, although in preventative medicine, there is never a 100 % guarantee. it does sound like Pauls case, is relatively unusual, regards David