I moved abroad when I was 18 so the age isn't what I'm the most worried. And if her mind is set to go you can't really change it, but you can maybe try to change her mind on where she goes. The thing that worries me the most is that she will not be independent. She will be working with her future husband's family. What happens to her job if she wants to break up? Plus her boyfriend has to go to army soon and stay there for a year. What will she do during that time?

Other thing that worries me is her education. Finland is just next to st. Petersburg. And there are trains going constantly between them. (I know, I live in Finland) Everyone speaks English here, it's a safe contry and it's one of the leading countries in the world when it comes to education. If she'd apply to study in one english taught bachelor degree (in Lappeenranta or Helsinki) and have her own apartment that way she wouldn't be quite as tied to her future husband but could still see him often. Finland is part of EU so travelling in Europe would be a lot easier and she could live her "European dream"