A kiss is perfectly legal. "Illicit s3xual activity", which is defined by the law as actual s3xual contact, would not be legal, unless 15 is the age of consent within that state. The laws really depend on the state in this case. One way or another, the kiss is legal, and it's also legal for them to date, they just can't engage in s3x.
It's not legal. I don't think anyone will do anything about one kiss between a 15-year-old and an 19-year-old, but it's still wrong. The parents could still press charges. You should warn your friend to stay away from minors. If things get any further, there will be trouble.