He only wants his visa

So my friend just got engaged today. And although this should be an exciting thing it's not. Let me begin by saying I'm in no way shape or form jealous. Read before you post your negative comments. So here's the thing. She's only met this guy for around 6/7 months now. Only really been serious for close to a month. When I asked her if she love him she hesitated to answer. She said that neither one of them had dropped the L word. To me that doesn't sound right or reasonable to even get married. Anyways she asked if I could be a witness and of course I said yes. But she doesn't seem all too happy about this. She's happy but not genuinely happy of this moment. I put the pieces together and realized that he only asked her to be his girlfriend right after DACA was announced it would be terminated. Now I'm Mexican as well as her now fiancé so I understand that everyone should have a shot at educating but I'm looking out for my friends feelings. I really feel like this guy doesn't love her at all and is only in it for the papers. Should I tell her how I'm feeling and to tell her it's okay to not go forward with the marriage? Or should I just keep quiet? Mind you she's head over heels for this guy so I know she'd be devastated if he did end up leaving after he got his citizenship