He's not frum because he wasn't brought up that way. He was never educated properly about the importance of dating jews. It's not his fault.
You're right, it's not his fault. It is the fault of those who stole away Judaism from him and stole away his heart and soul. This guilt goes back to the founders of Deform Judaism, and falls at the doorstep of every "Jewish" organization in the country. At the same time, there is always a chance for redemption. If you can somehow prevent him from intermarrying and over time show him the value of Judaism (show how great it has been in your life and how much you love it - but always careful not to overly "missionize" because that will turn people off), eventually he might realize how much you helped him and he might start to learn Torah or at least give his future (Jewish) kids a better Jewish education. This is all way more easily said than done.
But if you've already tried telling him straight up, and he didn't listen, then he is responsible. You tried. What I would say is to go a different route since straight talk didn't work. Go the psychological route. Make insinuations about the girl that she isn't smart enough for him, or isn't pretty enough, or isn't this or isn't that..... Just get him thinking. Don't say things like this directly but with sort of off-the-cuff comments..... where you are hinting at something without fully saying it. Things that are sort of laughed off, but later on he really will think about them and have doubts possibly... Just planting seeds. When they get in a fight or he meets someone new, these will come into play. And at the same time, while you are being subtle about breaking it up, you can of course never express approval in any way for this terrible situation. So you are basically neutral about it unless asked for approval which then you express how wrong it is, but otherwise you are sort of underhandedly messing with his psychology in regards to the girl without him knowing necessarily. That seems to me like a good strategy that has a chance of working. When you tell him not to and he doesn't listen, you can't really expect that straightforward way to work. That's my opinion. I am interpreting from what you say that you have already tried this route and it failed.
Yeah, he SEEMS happy but that is because he is getting some with this chick. Once it becomes physical, guys become blinded about the girl. And he can be just as happy with a different girl. Much more happy with a Jewish one. Well, good luck.
Topic: one of my best friends is dating a non-jew.What would you do? (Read 2975 times)