With neon lightsBlimey mate, that`s a tough one to land on you and it sounds dodgy as hell.
She`s a foreign national, he`s only getting married so she won`t have to leave the country when her visa expires and you`ve never met her !
There`s red flags going up all over the place.
If it was me, I`d run a mile.
Ps - you don`t know if he`s parted with any substantial sums of money in arranging all this or if he`s been paid to do it ?????

I guess you knew he was seeing someone and this just hasn't been dropped on you out of the blue? I know best mates don't have to live in each others pockets and can go weeks, months or even years without necessarily seeing or hearing from each other - life circumstances change and dictate this , I know - but this is a big deal and, whilst probably genuine and innocent, it is difficult not to have those doubts. I suspect you're primary concern revolves around whether your bezzie is making a huge mistake rather than if there is something illegal going on or the choice of wedding attire.
If he comes back to you and wants to understand why you've adopted the stance you have then I would simply tell him it's all a bit sudden, you haven't had time to take the news in and not even having been introduced and gotten to know the lass it's just making you uncomfortable. Explain that whilst not wanting to break your friendship bonds you don't really believe you can do justice to the role. If he knows and understands you as we on this forum have come to know each other and our own personal dilemmas I am sure he would not want to place unnecessary pressure upon you given your reaction.
I am sure it will resolve itself.