How do I get Dad to stop criticizing Mom?

I am caring for my parents who moved to Independent living that is close to me. They sold their home in a town they had lived in for 53 years. It has been a difficult transition for my dad. He has started to criticize what my mom wears to the dining room. He says that “all of the other ladies have changed what they wear with the seasons, but she is wearing the same old thing.” Mom is 90 and Dad will be 90 in Feb. He complains because Mom won’t make decisions, but when she does, he criticizes the decision or questions it. They both are depressed and I don’t know what to say to him to convince to stop criticizing Mom. She has mobility issues after having surgery for 3 herniated discs in 2015. She has never returned to the same level of mobility since then. She uses a walker everywhere she goes. Dad worried way to much about what other people think and not enough about what Mom needs. Any advice on here is appreciated.