I told my kids a number of times in their teens that:
- a great part of their success or failure in life will be determined by who they surround themselves with
- if they want to know what kind of person they are becoming, look at the 5 people they spend most of their time with
- a group of people’s behaviour usually settles to match the person in the group with the worst behaviour, and that it is a rare person whose behaviour uplifts a group 
- they should try to be a leader whose behaviour elevates the group, even if it means certain members drop them and the rest as friends 
- it’s better to be completely alone in the world for a while than to be in the company of one asshole

In the end, on this particular lesson, it comes down to the kid.  One of mine has heeded the advice. The other has not, and has paid the price repeatedly, sometimes dearly.