No, he does not have the right to change the locks. He does have the right to begin the process of eviction for your mother failing to pay her rent for March (and April).

The issue of her replacing the windows and kitchen cabinets and flooring and the landlord not addressing repair issues over the years is a separate issue from her failure to pay March rent. Doing one does not justify not paying rent in the eyes of the law UNLESS she and her landlord have an understanding that the work she has done is in place of her paying the rent (and it doesn't sound like this is the agreement).

Gail