Let the boy attempt to be a boy and decide later in life if being a girl is for him. For all we know the mother wanted a daughter and is forcing her son to be the daughter she never had. No child this young can know 100% their sexuality or gender expression. Let the child explore all his options before deciding so soon on one. Just like we ought not force him into the heterosexual lifestyle we shouldn't force him into the homosexual one either.
She's empowering him by letting him express himself harmlessly in a way that he feels. There's nothing inherently wrong with a boy wearing a dress--if girls can wear boy clothes and not get mocked, why can't a boy dress femininely if that's what he prefers? It's deeply rooted in traditional sexism, and the only way that will change is if people reject it and do what they like. Eventually, such misogynistic mindsets will wear off in our society.