Change in contract, is it allowed

Hi, I'm asking on behalf of a close friend as she has no idea if its allowed or not. Basically her husband was on a permanent contract which stated his hours were 40 per week with 1 Saturday a month. This went well in the beginning which is around 5 years ago. Roll onto last year and his daughter suffered a life changing illness which has rendered her paraplegic and needing a tracheostomy and tube feeding. She spent 16 months in hospital.

With this is mind her husband approached his boss about doing a 3 day week as they also have other children. Owner of the company agreed to this and adjusted his salary and contract accordingly. It did state in the new contract that he would have to cover leave of other employee's. Turned out he ended up working 6 days a week most weeks and not the 3 agreed on. He was paid for these days although 1 day a week was cash in hand. His hourly rate was incorrect and has been trying to get it rectified as per what was stated on his contract.

He went in to the owner today with his contract but the owner had drawn up a new contract stating that he was now to work 6 days a week, 46.5 hours and any extra hours as the company sees fit. The contract also states that the extra hours are unpaid. He's also supposed to get an hour unpaid lunch a day but as they don't close the store he ends up having to work through his lunch as there is no one else to cover.

My question is what rights as an employee does he have and are the company allowed to do this. The company have also held onto his old contract and ordered him to have the new one signed tomorrow.