Whether or not the narrator in Haruki Murakami's A Wild Sheep Chase was selfish when his girlfriend aids him in his hunt for the mysterious sheep will ultimately be up for you to decide. However, we can help you figure out ways to think about this question.
First, let's look at the narrator himself. Is this a person you'd want to help out? Is this a person who seems like they'd help you if you were in a pinch? Remember, our narrator is an advertising executive whose wife has just left him. Such traits don't exactly provoke our sympathy or empathy. The characterization leads us to believe that her help will be more for his own benefit than for hers. After all, he's the one getting lots of money from mysterious people.
Let's also look at the girlfriend. If it's selfish for him to have her help, doesn't that imply that there's other, more important things she'd rather be doing? What does the girlfriend do? According to our narrator, she was "a part-time proofreader for a small publishing house, a commercial model specializing in ear shots, and a call girl in a discreet, intimate, friends-only club."
None of these occupations seem to really interest his girlfriend. They don't appear to be great passions of hers. Again, it doesn’t seem like she's giving up anything meaningful by joining her boyfriend on his search.