What is the ruling on my daughter working in a private company whose owners are suspected of dealing in expired goods or evading taxes, and is her salary from it considered unlawful wealth?

If the girl's work in the company is permissible, then her salary is halal because it is in exchange for her permissible work. The sin of dealing with expired materials falls upon those in charge of the company and the employees directly involved in that work, and does not affect someone who works in another permissible field.