Actually, if he has a permanent residence (aka Green Card) he can join the Marines as an enlistedman. If he can become a sniper right away, I don't know. What I also know is that you can apply for citizenship right after you complete boot camp.
He will have to ask his recruiter about possibilities of becoming a scout sniper later on after he gets his citizenship (if such is required for scout sniper training and qualifications) while he is serving as a 0311 (infantryman). I think that is how it works but calling a few recruiters and other Marines would be a bit better. I know a former USMC Combat Engineer who was an excellent shot, and though not a sniper himself, trained on the M-82.
But he's got to remember that becoming a sniper is some unreal stuff and sometimes if you are unlucky, you may just never get the chance. I was in a trials team made up of the 50 best shots in our brigade of about 3000 and of course even then I wasn't any kind of sniper. Even getting to that level isn't exactly a walk in the park.
I think our friend here should focus more on physical and mental fitness and on becoming a good Marine, rather than any of the fancy stuff. If he makes scout sniper, great, if not, there are plenty of other things to play with.
Try this link below:
http://usmilitary.about.com/od/enlistedjo2/a/8541.htm
it doesn't say whether or not you need to be a citizen... but the site actually says that for all MOS that require citizenship. So I think maybe you are alright.
Good luck shooting expert, over and over and over and over though.