This is quite a dilemma regarding what should be the proper thing to do versus the right thing to do. There will always be the police of morality who will preach you not to do unethical or illegal things but these same people will not hesitate to cheat in exams, file for asylum, fake their resume, have someone else take their interview for them or sleep with their friends gf wife if they got a chance.

You can always test the waters by getting some opinions but you should feel free to do what would be the best thing to do. Having your gf 'marry' your friend to come to the US does not hurt anyone and that's the basis of ethics I try to value most of the time if it comes down to that. In this instance, noone is hurt so no need to feel bad about it if it works out.