Difficult question, 'cause if someones really addicted to the shit they're gonna be lying about a lot more than being addicted. I dated someone who was addicted to coke/crack/heroin for 6 months, refused to go to rehab, lying and shit got out of control until he got arrested. Got out, stayed clean for a while, went through drug school, and just last week, he died of a heroin overdose.
I don't know what else to say. I tried to stop him, everyone did, but you've got to get them before it gets out of control. I didn't know he was even doing the shit as much as he was, and thats how he got hooked.
Tell them to be careful man, hang in there.