Your friend is taking a great risk dealing drugs, depending on how often and what kinds of drugs he sells. You can call him to account for his behaviour and tell him that he puts himself and other people at risk. You can also call him to account for the risks he is causing you and tell him that you want him to stop. Your friend has two options then: to quit or to continue. If he chooses to continue, it is up to you what to do.

You could break up with him and tell him you will not get back with him until he stops dealing. Then the choice is up to him: continue dealing or continue the relationship. If he doesn't stop, ask yourself what you mean to him. You can also offer him to seek help together. If this drug usage is illegal according to the laws of your country, you might be legally obliged to go to the police.