No you can't. The official explanation is that the

relationship interfers with equal treatment to other students. But the unofficial reason which is more true is that no one want to deal with sexual harasement lawsuit.

But people can get around it. I have seen advisor marris his graduate student here in US universities. If your friend can offered a sabbatical leave and get married, then it will be OK. I am not a lawyer but my comment is based on cases I've seen during the years.