He needs to check his contract that he signed with his employer when he started.
Business's can change your hours or any part of your contract with an agreed period of notice that should be documented in your contract.
For example our company changed some staffs working days. The staff that didn't want to change were told that they were being given 12 weeks notice
I think even if it doesn't state it in your contract, as long as a reasonable time frame is given then they are not doing anything wrong