I think the issue is with the company that is refusing your daughter's requests to send the mail to a different address, not her employer.
He has no right not to forward mail to your daughter but it is only hearsay that this has happened.
Regarding the employer, I suspect he is opening only her mail because she is only one not resident that has mail delivered to the hotel. It is impossible for an employer to know the contents of a letter without opening it and it is reasonable to assume that mail addressed to the hotel is business mail.
Your daughter should make sure her address has been changed by all the companies that she deals with, including her GP, dentist and any hospital she may currently be receiving treatment from.
Is your daughter being bullied by the employer in any other way?