What do you do when your cousin won't listen to you about a guy you know personally?

So I have this cousin and I gave this guy who used to be my friend her sn on yahoo, so they could talk. So anyways they been talking for about 2 weeks I think, I'm not sure. But I feel they are taking things to fast for my taste and the fact I'm trying to look out for her. He has told her he wants to get eloped and move in with each other, but they haven't even met in person and he has already told her "he's in love with her". And I don't buy that bs at all and I been trying to tell her that he wants but she doesn't want to listen to me. Do I let her make her first big mistake as a adult or do I keep interfering?