How do I convince my nephew that his girlfriend isn’t worth it?

So my nephew is a senior in high school. And ever since he had this girlfriend of his, I’m worried about him. First of all, he wants to go to the marines instead of college. And I think it’s because of his girlfriend. I think he just wants to impress people by showing how “manly” he is. I’m a first responder, and I told him that the marines don’t make that much. And there’s quite a bit of guys who come from the marines, and they work with me. They also told me how little they make. So I told my nephew to just become a first responder instead. You’ll skip that part of wasting years. And he can make more right away. He’s also smart, so he has potential in college. But he wants to “get skills.” I told him that if you don’t already like listening to your dad (my older brother), why do you want to be yelled at all the time in the marines? He didn’t say anything. So anyways, he only had this girlfriend of his for a few months, and he recently showed my family his promise ring. I’m worried that he’s going through mental illness. Because it’s his first girlfriend ever, and he’s crazy over her. My brother says he spends all his time at his gf’s house, 24/7. And this girl for reference, has had multiple boyfriends. And has mental issues (depression). And her family is dirt poor to say the least. And before you say anything, my family isn’t rich, why actually grew up super poor, but that’s why we don’t want him to struggle as much as us to get to where we are now. We suffered so much. Also, this girl has done drugs. Has no real future, and doesn’t even drive yet. I don’t want my nephew around her. And my family is theorizing that she probably wants to be a stay at home wife. And that’s why she likes my nephew, because he wants to provide for them.

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I told him multiple stories of people I know on why you shouldn’t get married so young, and how 99% of the time they end up in divorce, or they hate their lives because they got married so young. And how they can’t do anything because of kids, or how they now have to go through the divorce process. Or how they’re stuck with someone who’s not good for them, and they stick it out because they’re married now. I told him not to take her seriously because you’ll
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End up meeting so many girls later in life. And once high school ends people go on their own path.
How do I convince my nephew that his girlfriend isn’t worth it?
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