There are many cards that could indicate drug use. Different cards for different drugs, different scenarios, different intensities....so much of it is the feeling you get when you look at the card.
Additionally, I don't actually believe it's right to read cards to discover your friend's past without him knowing. If he has used drugs in his past and has chosen not to tell you, that's up to him. It is not your place to be delving into a past he has chosen not to reveal to you.
That said, I find these questions interesting as a way for me to study my cards, so offhand, here is what I have come up with. Hopefully, it won't be used for ill.
For addiction:
The Lovers - So many people have a love/hate relationship with their drugs. It makes them feel loved and lovely and powerful in a way that few things (if anything else) ever can. They cling to it for hope, feeling, and euphoria in those fleeting moments. It is their lover, in substance if not in physical human form. But the Lovers is about choice as much as love. Every time an addict picks up their drug again, they make a conscious choice. When people are addicted, they often know the consequences of their drug use, but they don't care. The need, the addiction blinds them. If you are seeing an unhealthy aspect in The Lovers card when you read about drug use, it could be indicating addiction.
The Devil - Also so much about addiction. I often think The Devil and The Lovers are incredibly similar. In The Lovers, the person makes the conscious choice to take a drug because they love the feeling. But I think in The Devil, they feel that it is beyond their control. They really want to stop, they know it's harming them and it's gone too far, but they find themselves coming back to the drug again and again. While both The Lovers and The Devil can be about addiction, The Devil might be a more severe or realized addiction. In The Devil, the person wants to escape from it, but in The Lovers, they still haven't admitted that it's such a problem.
The 7 of Cups - This card tends to come up when someone can't stop thinking about something, or isn't accepting reality. Therefore, it could be about their obsession and need for the drug, or their inability to accept that they have a problem. However, this card could be more about experimentation (with lighter drugs or hallucinogens) rather than actual addiction. Alternatively, because a lot of former addicts say they never fully "overcome" the craving, this card could indicate the fantasies they occasionally have about returning to the drug, or withdrawal shortly after quitting a drug.
9 of Swords -- More about someone trying to overcome addiction. They are trying to cut the drug out of their life, but it's all they can think about. They can't sleep, they can't concentrate on anything, they feel guilty and know it's bad and wonder if they've ruined their lives...but they are fighting it. Moping around and feeling miserable, perhaps, but refraining.
The Tower - Upright, I could see this card as representative of someone going cold turkey on an addiction. It's probably the most sudden and shocking way to get off a drug, but some people think it's best that way. Like ripping a band-aid right off.
The 7 of Wands -- This person may have had some bad habits and mild addictions, but he has decided he is above it and strong enough to fight it off. This is more likely in someone who has only experimented a few times, but doesn't like the path he is going down. I don't think anyone who has gotten into the depths of an addiction would be able to fight it off this way early on.
Other drug scenarios:
The King of Chalices (reversed) -- I could see this as someone who still occasionally indulges in their drug of choice.
The Moon -- Experimentation with hallucinogens or other mild drugs for the rush and experience, but not representative of addiction or common usage.
10 of Swords -- Possible overdose at some point. (Remember, overdoes doesn't always mean death.)
I could see The Magician as representative of a drug dealer, and the 6 of Pentacles as a dealer/user relationship.