its not really any of your business so you shouldn't be asking for advice.
sorry if thats blunt

Ive known a lot of things a lot worse were happening to my friends, and I recognize that people have been in your position wondering what to do about me several times, you have to realize you aren't god, you dont have the right to deny anyone freedom, MOST of all your friends. On top of that its going to hurt her if you try, I know I wouldn't be happy if my friends were trying to tell me what to do with life. Its her choice who she dates.. the best thing I think you can do is just be her friend, tell her what you think without getting in her face, and be there for her if she does get hurt.

well since she is a minor, advise her about the penalties her boyfriend could get in if she engaged in coitus. I have a hard time believing that a relationship between 14 and 19 year olds could be endearing, or anyway meaningful but then again, that's just an opinion.

I really dont see how a 19 year old could really be interested in a 14 year old girl.

I guess the only thing you could do is tell her that he could get in trouble for doing some things with her.

In most jurisdictions, that would more than likely be illegal. However, I wouldn't say anything to any authorities, because that could chance your relationship with your friend. You could try talking to them, let them know you think what they're doing is wrong, and probably illegal. This might not do much, but it's worth a shot. That's a huge gap, and I think the maturity levels are too far apart. They know the risk, so sometimes you just have to step back and allow things to take their course.

I think it's OK if she likes him.
what about her parents? Are they OK with it?

i have known many relationships like this that have worked well, like one of my classmates who was like 15-17 at the time, and starting going out with a guy who was like 21-23 and they have been together for about 3 or 4 years, luckily i knew the guy so i knew he wouldnt jerk her around, try and get to know him, see what kind of guy he is like, alot of guys can be nothing but respectful in situations like this, do the parents know?? because if they are ok with it then you should be too, all you can do is tell her to be careful. but basically its her life and its her mistake to make if it ends badly, she will learn and mature from it.

Sex.

Or maybe he is too immature to date women girls his own age.