He experiences a sort of physical/emotional pleasure. However, the habit of taking heroin harms his overall health, and his overall good, so he cannot be truly happy. Happy people do not destroy themselves. In fact, the drug addict is neither happy nor free, but is simply a slave to sin and vice.
It's only when they're not fulfilling their desire for drugs that they're not happy.
In all senses of the word.
The pleasure of the drug use overrides any other considerations. Even continued life may not be as pleasurable as being high.
Making oneself happy can be quite self-destructive, but that does not mean that such a person is not happy.