Those who involve themselves in affairs with another's spouse, lacks understanding and is destroying their own soul (Proverbs 6:32). The marriage bond is a holy union joined by God, and those who tarnish that bond by becoming intimate with the spouse of another, are sinning not only against self, and the person's spouse, but they are also sinning against God (Genesis 39:7-10, Hebrews 13:4, 1 Corinthians 6:18, Matthew 19:6).
The best thing to do is never get involved in an affair; for no affair is worth spending eternity in a burning hell, and that is sure where one will go if repentance isn't done (1 Cor. 6:9-10). Besides, chances are the person won't leave their marriage, and even if they did leave their marriage, the law of God won't permit one to marry the spouse of another (Matt. 14:4, 1 Cor. 5:1, Romans 7:1-4, Matt. 19:9). So it's a losing proposition.
If one is having an affair with the spouse of another, you need to stop it, and stop it now. You are involved in a terrible thing, and you need to do the right thing. Someone else's spouse is not your spouse, and can never be your spouse, so leave them alone and find someone who is eligible to be with you. And we do mean someone God would be happy to see you with (Prov 18:22).
Hope to hear from you or to see you at the services.