If the person says you can, then you can use his first name. But using his first name without permission is rude. And you can't ask for it, you have wait for him or her to offer.

Even then, be very careful if your friend is (by Korean standards) of a higher status. That is, if he's older or richer or has a nicer job (which for is just about everybody except kids, bucko), always use Mr. or Mrs. in public, never the first name.

Now here's an intriguing difference. Married women don't take their husbands' family names. This was why Margaret was initially confused by Mrs. Lee and Mrs. Kim. At first they were trying to westernize their married names, but then they forgot to keep up the act.