Hi. Its heartening that you sre showing so much concern for your friend. Addiction of any substance is a difficult situation and its very difficult if the person addocted himself is not willing to quit. Your friend is dependent on thinner which falls under category of volatile solvents which itself causes lots of harm and also predisposes to addiction of other substances. The best way out will be to start building up motivation of your friend. Once he is motivated, it will be far easier job. You can ask him to comsult a psychiatrist for the motivational therapy and also start treatment for his dependence simultaneously.