Humans routinely socially evaluate others not only following direct interactions with them but also based on others' interactions with third parties. In other species, 'eavesdropping' on third-party interactions is often used to gain information about foraging or mating opportunities, or others individuals' aggressiveness or fighting ability. However, image scoring for potential cooperativeness is less well studied. Here we ask whether a non-human primate species, tufted capuchin monkeys (Cebus apella), socially evaluates humans after witnessing third-party interactions involving a helpful intervention or failure to help. We find that the monkeys accept food less frequently from those who persistently reject another's requests for help. This negative social evaluation effect is robust across conditions, and tightly linked to explicit refusal to help. Evaluation of potential helpfulness based on third-party interactions may thus not be unique to humans.