Hard to say what id do.
But hypothetically putting myself in his shoes if that was my fiancé I would have confronted her with the evidence, express my displeasure and dissapointment. Try to make her feel the weight of fucking up our relationship make her feel like the worse person in the world for ruining our relationship. But considering she already got a reputation for doing fucked up shit, maybe if I kept it secret that I knew she cheated and just kicked her to the curb without closure. Leave that hoe guessing where she fucked up at.