When I was younger, I thought just because someone was a Christian that they just agreed with all the stuff they should. In today's world, that's far from true. Sometimes this still bugs me a lot, but there's where I have to remember: we're all on different levels in our faith. Here's what I've learned from that.
You have to be a light. Sometimes you can't just tell people things, you have to show them. Believe me, people notice. Don't give up, God will use you.
If the time is right, confront out of love. Confronting without the love is dangerous, and loving without the confronting is, too. You have to find a balance. Pray that God will help you find the right time to confront - He will. Maybe it will be adding a comment to a conversation or sharing your story. He'll let you know.
Leave it to God. Pray to God about the people and their struggles. He's listening, and know that ultimately God will take care of them.
Worry about yourself. You're not perfect either. Ask God to show you the areas YOU need to improve on, and focus on doing that.
Appreciate the person's good qualities. Everyone has them, and if you think they don't, YOU'RE NOT LOOKING HARD ENOUGH. Enjoy those good qualities, too.
Recognize their strength and strong areas. And tell them how much you admire their strength there and what not. How you see life is a choice. How you see people is a choice.