Friend - 16 - Considering children and marriage.

Hey. My friend is 16, and her and her boyfriend have known eachother for 8 months, and have been going out for 4. Since before they were going out, he talks about marriage, and having kids with her. At the moment, I believe he’s looking for an engagmement ring for her. They are constantly talking about their future together, and what their childrens’ names will be. He’s 18. They always have to be with eachother, and they’re really needy as far as being with eachother goes. According to his best friend, he’s discussed marriage with a few of his ex girlfriends, but my friend is the one that seems to mean the most to him. It just worries me because they moved so fast, he’s her first boyfriend, and we try to make them understand that they’re looking extremely far into the future with only a few months of being with eachother, but they just get upset and tell us that if they work at it, it will work, and they’ll be happy. I’m scared she’s going to get her heart broken if they happen to break up. Any advice?