If you really care for your friend and can afford it, buy him a ticket to Peru.
Tell him that you thought about the situation, and realized you are not the one to take decisions for him. Tell him you are supporting him in whatever he decides, and you are always there for him, to make it up for doubting his decisions, you bought a ticket, so he can go and see the girl. Maybe tell him you can't see him just spending his days next to the computer, and you want him to be happy and finally see the girl.
Be careful when wording it, if you say "You want him to make sure the girl is really what he thinks she is" you might make him defensive and suspicious. Sure, that is your goal, but don't say it.
Kidnapping/kidney stealing risk aside, if gets to see the girl face-to-face, there's a few scenarios I can think of:
1) The girl is just as he imagined, they both really like each other, and know they fell in love or whatever. Be happy, your friend found someone he really likes, and at least he got to kiss/bang her. He will love you for this.
2) He finds out the girl is actually fat, obnoxious village slut that has been milking him, or something like that. His illusion of the perfect girl gets shattered.
2a) Some doubts will enter his mind. It might take awhile, but the seed of dissatisfaction will be planted and he will likely break up with her after some time once he comes back, when he realizes she's nothing special, or that he can do better.
2b) He will drop her right there because he realizes it all was a lie he manufactured himself, spends a couple of days in Peru crying, then comes back to US and thanks you.
3) She plays him well, he comes back home happy, after a while girl moves to US and just milks him for money/visa, or whatever her original plan was. Which would happen anyway, if it were to happen.
Of course, a ticket to Peru might cost you a bit of $, but that's the question of what is more important to you.