Symbolism, Imagery, Allegory
The popular drug is an image of pleasure, forgetfulness, and escape. When Fowler takes it, which is frequently, he loses his sexual desire, rendering him even more uninvolved than usual.
Preparing pipes for Fowler seems to be one of Phuong's regular activities and the closest thing she has to a job. After she rejects Pyle's offer of marriage, she offers him a pipe (2.1.142), as if that's the natural course of action in the circumstances. Pyle declines, and she asks Fowler if he's like one. He accepts.
Why does Greene associate Phuong with opium? Perhaps because, to Fowler, she is a pleasure and an escape, and little more besides.