Final answer:
The friend is motivated by a purposive incentive, joining the labor union out of a moral and social drive to address the perceived unfairness in their wages, rather than merely for tangible benefits.
Step-by-step explanation:
The type of incentive that appears to be motivating your friend to participate in the labor union and campaign for higher wages is a purposive incentive. This means that your friend is driven by a sense of purpose, passion for a just cause, and a sense of right and wrong in relation to the work they do compared to the compensation they receive.
A purposive incentive is what compels individuals to join groups in order to support a cause or an issue they care deeply about. Such incentives appeal to someone's concern about advancing their beliefs and strive for social justice, rather than just reaping material benefits. In your friend's case, the injustice they feel about their earnings in comparison to their effort is a strong motivator, and thus, the social and moral rewards of fighting for fair pay through the labor union fulfill their purposive aspiration.