Why you should not assume someone is lucky or declare to someone he is lucky!
1) Luck is, most of the time, a consequence of hard work and effort or a good/thoughtful decision and the person hearing it is not going to like it if the situation he is in is attributed to just coincidence (which is what luck mostly means)
2) Calling someone lucky typically, err, hints at jealousy (even if just a little) whether we like it or not. Or like one of my friends said, it means you probably don't deserve what you got!
3) You may be commenting on one particular aspect of his life but there may be a lot of other problems in his life that may not even be compensated for by this apparent "stroke of luck"
4) There are so many better ways of responding when someone is going through a nice phase or a good thing has happened to them - You can say "That is great!", "This is cool!", "Happy for you!" or even "You are blessed" etc but of course you must mean it. ;-)