The word “tart” is a derogatory term for a sex worker, but it can also be used more broadly to refer to somebody who behaves in an overtly sexual way. As such, the fact that Joe calls Laurie a “tart” doesn’t necessarily confirm that she’s a sex worker, though it
does make this possibility seem even more likely than it already did. Either way, both Laurie and Joe suggest that Anna has behaved somewhat hypocritically by entering into an obviously transactional relationship and then trying to back out at the last minute. In other words, they think Anna should have known that everyone expected her to have sex in exchange for some money and a nice dinner.