Salaam,
It actually depends on how old they are. If they feel that they really do love eachother and are ready to take it to the next and final step, then I would advise them to get married. I remember recently, a good friend of mine whom I'd know since four years had feelings for me and I was just waiting for him to come forward with it. I really do not like being told lies. So he decided to tell one of my other friends, and she told me and asked me if I had wanted to go out with him. She co-erced me into dating him, but first of all I had no feelings for him, and secondly, it is haraam and only leads to heartbreak if you don't intentionally plan on marrying them. The friend wouldn't talk to me for about a few months, but now understands why. So..If I were you, I'd have a sit down with them and ask them some really deep questions and get a sense of whether they really are ready for eachother. They should also talk to their parents about it. Inshallah, I hope they do well and find what's right for them..
W/Salaam