I don't think I could be friends with someone who would marry simply for citizenship. I guess it's one thing if the guy understands that is what she's after and is happy enough to go along with it, but it's completely different if she is not being upfront with him about it. That's just plain manipulative, and that's something I could not tolerate in a friend.
I also think you need a pretty strong relationship to be able to convince the relevant authorities that the couple is "for real". I had a friend who partnered up with an English guy and they had to jump through hoops with the authorities for him to be able to stay on here. I was one of their character references and the questions that I got asked about their relationship were pretty invasive.