Greetings Calluna,
Wow! You've been getting some pretty good advice here!
If I could add my bit, as a father of 11...
Well, first, sister, best summed up....JESUS.
He's really the beginning and end of it all...remember the saying...WWJD, what would Jesus do?
Having said that, well, Jesus to us is a relationship, so looking at what He told us, to love others too, in the same breath as loving God, [the two go together...can't do one and not the other] our position with others is based on relationship...meaning, that being a friend who loves, not the fleshy side of a person, but the person inside the flesh, who they are, yes, loving the person with God's love is the medicine to all lifes problems. Sounds a bit simplified, but it's true. Your son is a living soul and so is his friend.
Be their friend first....the rest will follow. If you want them to hear anything you say, meaning if you want them to know what it is you want them to know, they will begin by reading who you are, looking and listening to the person they see given the way they are presently looking at life and the world. Just love them...do a bit of Jesus and look way beyond the outside appearance and behaviour and look and see that they are precious souls worthy of every bit of divine love imaginable. Let your heart be elated with their company and speak encouragement and love, have fun with them as your friends...relate to who they are, not what they are. Nothing is so potent than God's love, and He has given it to you, and your only part is to let it flow, remembering it is His love and therefore can and will do what He desires it to do. It is His job to speak to them, and He does this with/by/because of His love, which is one with His Spirit, one with His will.
If we look at the clothing it will get up our nose. If the clothing improved, we would move on to the next thing we don't approve of, and then the next.....and we are not called to this.
If you do say something to either of them...say it once. Do not repeat it!
I have found over and again that be being a friend, and giving thanks to God in Jesus Christ, He sorts it all out...and He is good at it, and, quick.
Their souls need to be loved. That's all. Love love love love love....in fact, the more they feel loved by you, the more they will want to please you.
Remember this....God is in control and He has plans for the children. This girl is now in your life and God has put her there...why? To improve her dress sense? I don't think so!!!
Hope you understand all this..as I said first, Jesus.
Shine your light. It makes a huge difference to those who can't see too well and are in this dark world.
Sister, give it all to the Lord, and trust Him to sort it all out...He will!
And, thank you Jesus for Calluna and ALL she is and ALL she has on her plate right now...you are wonderful Lord and thankyou that you are doing great things and only have good in mind, planned for Calluna and her family...bless you Lord...thankyou for being a friend!
Bless you,
Br. Bear