If you really want to say "I envy you" in that exact same way, to a friend of course, then you could probably say: 널 부러워해! or 널 질투해! which both essentially mean "I envy you". But if you want to say something in the lines of "Good for you", you could say: 잘됐군요 or 그거 좋겠어.
굿럭~^^