No, it's cowardly to go behind a man's back to gossip to his lover. If you have designs on her yourself, don't do it, you'll be inveigled in a curse. There is no good end to your plot here. Amend your plot I suggest to either tell the man and woman both to break up because they are committing fornication and adultery and they are not to have more lovers, or elsewise, if you are so deeply concerned for the woman altruistically and not with a devilish plot in mind, then there should be no problem making your accusations about the man with him present to defend himself.