Sounds like your daughter is telling the truth, but you can't control what happens at dad's house. If he is still seeing this person there is not much that you can do. Maybe counseling would help you get past the hurt and anger of the past and put it to rest. I know it must have been a terrible blow to her your daughter call her mother, but it's more important to keep an stable attitude toward it and not put your daughter in the middle of and adult fight. Think of letting bygones be bygones. Your anger is only hurting yourself and maybe your daughter.