Priests are not allowed to withhold Holy Communion unless the person in question is a public sinner (or in some other way publicly excluded from the life of the Church). By the way, 'public sinner' is a technical term. Even if the sinner in question told you, or even several others, it does not make them a public sinner. The priest will make that judgment. Depending on the situation, you may even be risking the sin of detraction by spreading that information.