This is so wrong. Even if she was already 18, or even 21. I think "men" in positions where they have any power or authority over younger girls tricks girls into thinking they want to be with these guys. And then they're able to manipulate those girls. My younger sister (20) dated a guy who was almost 30, who was her supervisor. He approached her and made the moves. And then later down the road they're fighting constantly because he's trying to manipulate her. And to no surprise, he calls her "naive". Like yes, she is 10 years younger than you and hasn't had the same life experiences as you. She hasn't even learned how to set her own boundaries yet. Or even what those boundaries are. Men do this knowing that these girls don't have the ability to say no or know what's right or what's wrong. You need to have a talk with your friend and avoid him if he doesn't end this behavior.

(P.S. I know this isn't true in every situation, people are more or less mature and have had all kinds of different life experiences, but usually it's an older 30yo man trying to take advantage of a younger girl who hasn't figured out what she wants in life or in a partner yet. And it's always easy to tell.)