Never. I don't want to actually make their decisions for them but I will absolutely weigh in on what I think. If my child can't trust me to always voice what I think is in their best interests who can they trust? I will say what I think stand back and let them choose.
My kids are now 11 & 12 and I absolutely talk to them about the different sorts of people in their world. Good friends, allies, acquaintances, bad friends, manipulative friends, frenemies, abusive relationships, loving relationships... all of it, I say what I see. Still, I always make sure to close with, "I won't tell you who you can or can't be friends with, I trust you. You're a smart person with good judgment." Then they get a kiss and a hug

I've been ending these sorts of conversations with my kids like this as far back as I can remember. To my mind, saying what I say gives them back the power. Of course, they always have the power over themselves anyway but I think it's important to acknowledge that I know it and respect it, KWIM
I don't expect to change when they get older, the problems will change but the conversations probably won't. Good luck, parenting is a really hard job