If you realised that a friend of yours was self-destructing? Maybe drinking too much ( enough to get alcohol poisoning one of these days) or meeting shady characters in dives you tend to drive a little faster when you go past or using drugs or having unprotected sex with anything that has a pulse without condoms or staying in an abusive relationship.
You've already tried talking to him/ and giving unsolicited advice and he nodded his head, told you how wise you are and continued with the self-destructive behaviour.
1. Initiate an intervention with his family
2. Do nothing ( this person is an adult and interfering always gets messy. And who needs the trouble, when your own life has its own challenges anyway?)
3. Tell your friend's family what he's up to, so they can decide what to do ( or not do) to help
4. Do something else I didn't think of when giving the above choices - Please explain what that omething else would be