How Do You Talk to Someone’s Parents About Their Drug Habit?

My brother and his girlfriend are both addicted to drugs. My brother is currently in a 90 day rehab program and has been clean the whole time and will be done soon. His girlfriend initially never showed any signs of drug abuse, till just recently we caught sight of her in a tank top with fresh track marks all over her. She currently has custody of my nephew, and I’m not happy about it. My question is, I would like to either call or visit her mom and dad and fill them in on what I saw, and voice my concerns, but i don’t know how to start the conversation. They don’t much care for my family since my brother is viewed as the “druggy” that ruined their daughter’s lives. But i think its very important that they are aware of what she’s doing to herself, and possibly doing while my 2 year old nephew is around.

Has anyone ever had to do that? How do i break the ice?