I didn’t say you’re not spiritual, just not mature in years nor strong enough in your Yiddishkeit (the example I used was Rebbetizin Jungreis) which only comes with age, fighting nisyonos, learning halachos and hashkafa, loads of siyata diShmaya — and preferably marriage to take away your gullibility.
You are not an unbiased bystander to judge if you were effected or not. But even if you’re right — this boy knows that what he’s doing is wrong — it’s not a simple matter to dissuade him for someone inexperienced in dealing with OTD kids.
You mentioned he has sisters, do you think they haven’t tried everything to bring him back? Most likely if you mention anything to him he’ll either become confrontational or invite you to join them. As a frum girl you CAN NOT OFFER HIM WHAT THESE GIRLS ARE OFFERING! I really don’t know why you’re looking for trouble. If you feel guilty doing nothing ask a shaila if you should notify his parents or siblings who he’s he’s going out with.