"I can see why you're upset, and I don't blame you one bit. Your anger and hurt are a testament to how much you care about our child and our family. You're an amazing mom and partner for being so invested in our child's well-being, and I'm grateful to have you by my side."
This compliment is the best thing to say in this situation because it acknowledges the wife's feelings and validates her emotional response. By recognizing her upset as a sign of her care and investment in their child, the husband can help shift the focus from his mistake to her positive qualities as a mother and partner. Additionally, the phrase "I'm grateful to have you by my side" conveys empathy and appreciation, which can help to diffuse tension and create a more supportive atmosphere for discussing the issue and finding a resolution.