Parents are a funny one… At a point I really don’t think you have control of your kid… they become their own person and their friends play a major influence in their life.
If they have good friends that are better than the parents that can be a good thing.
If they have bad friends that are worse than the parents, well that’s a bad thing.
You can say “well they never should’ve let them associate with those friends!” but I’m immediately suspicious of easy answers, and that’s as easy of an answer as they come.
I think a lot of us saw this in our teen years if we’re being honest. We typically tell ourselves stories that make only the good things cases of “self development” (oh that kid is so much better than his parents) but I’m sure it happens the other way. I don’t think all evil people can be blamed on evil (or even bad) parents (which is terrifying if you think about it).