Leaving Las Vegas, with Nicholas Cage... It's a very good movie with an amazing story. It shows the feelings of the actual addict, and it shows the feelings and frustrations of the person who loves the addict and wants to help.

There's a few other movies I'd recommend but most are mostly from the drug addicts prospective, but it does give you a good look at why they use, how they use, and what drugs/alcohol can do to them.

Requim (spelling?) For A Dream
Spun
The Basketball Diaries
There's also a documentry if you can find it called Black Tar Heroin. If you can find it, I'd strongly suggest watching it. It's hard for me to watch it now without thinking back to my 'old' days, but it's truely a good documentry on learning how to understand the usage of drugs.
Blow
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Those are the ones I can think of off the top of my head at the moment. I hope that is at least a little helpful. Although they don't really go back to parents addictions that leads to their childrens addiction (not all those films anyway), most have some sort of abuse by the parent which has lead them to drugs.

I commend you for caring so much about your friend. That's awesome!