If she was his client. Is there not any legal ramifications of them having an affair?
I would expose this affair to everyone very quickly. I would send a regestered letter to this lawyers firm about his actions. He is scum. He needs to face up to his actions. I would not even talk to him. You know all you need to know about him. He is a liar, does not respect vows, does not respect you and your children.
I would then re expose this to her family , your family and some very close friends who will support the family. Support the wife. But will not support the adultery in any shape or form.
I would also contact his exwife and let her know what is happening as well.
I would put some very strong boundaries down about this OM and your children. I would also get the support of her MIL in this.
I would also keep the relationship with all friends and family going. Smother her out of options for when OM is too busy for her. Let her have time to herself.
I would shore up all finances. I would also look into her paying child support and financial support while you maintain the household.