Even though you say you are not going to “drop” your friends, you may end up pushing them away regardless if you start to give them mussar. Right now they just need a lot of love. Show them that you will support them no matter what, and then after a while you can gently start to introduce mussar a little at a time. I would just hate for you to lose them over this because of overzealous tochacha.
Show respect. And be a role model that they need. This means don’t give them tochacha here and there. Actions speak louder than words. The closer you keep them, the more they will feed off your actions and see that you are a Frum Jew, you aren’t “rude” to people like them and you are there for them every step of the way. They will Bezras Hashem come out even stronger in their Yiddishkeit because of this.
When you say at risk and off the derech do you mean less religious or do you mean doing drugs and/or other dangerous behavior that even secular people disapprove of?