What is the best way to help 13 year old daughter understand why we have to rehome one of our 4 dogs?

Our 1 year old bullmastiff is having an awful time with housebreaking. We have 4 dogs so know the training very well. She was doing great for awhile then we moved to a new house. Now every day she is pooping and peeing in the house. Once she even pooped right on the front porch. She is let outside every hour including twice during the night. She just is not learning. Very close friends of ours may take her. She will be with another big dog outside. I think this is a better arrangement for her. My daughter, of course, is upset. She has an amazing way of guilting me into changing my mind. How can I help her understand and feel better about the decision?