Ask her/him if they are sure they want to do this. "Are you sure this is the right guy/girl for you? Do you really love them? You should think about before you actually mary. You don't want to get a divorce later on down the line. Think it over. You'll thank me some day."
Talking to them in person is probably better because then they'll get to see you're face. But try not to be rude because you don't want to lose a friendship. You may also want to have them talk to you for some time; sort of venting their feelings. Then maybe They'll realize that what they're doing is a bad choice.
GOOD LUCK!