At that age, not ok. Later in life, completely acceptable. That's the age difference between my girl and I, but we didn't meet until I was 26 & in grad school and she was a 22 year old senior in college. Or maybe I was 25 and she was 21....either way....different ball game.
Not only is what he doing now morally wrong, it's also illegal. (Assuming that they sleep together as that's what happens in all relationships, right? And they're not from some fucked up state like Kentucky or something.)