When it comes to describing a short-lived romantic relationship, there are plenty of synonyms for the phrase "has a fling." Some possible options include: "has a brief affair," "has a dalliance," "has a tryst," "has a one-night stand," "has a casual encounter," "has a hookup," "has a temporary romance," "has a romantic interlude," "has a momentary fling," and "has a short-term relationship." All of these phrases convey the idea of a relatively short and perhaps somewhat casual romantic connection, without necessarily implying any kind of long-term or serious commitment between the parties involved.