Yeah, what they said. "Good for you" should be translated literally.

However, like all human language, it can be said sarcastically or honestly, depending on the context. If you had no reason to think he was being sarcastic, then he meant it honestly.

Don't know if you ever got the definition of "once in a blue moon" or not?

A "Blue Moon" is a full moon appearing twice in the same calendar month. It is a rare event. So "once in a blue moon" means something that happens rarely.