no they can't change his salary without his agreement - he need simply refuse the reduction to his wage and then if they still go on to reduce it he can threaten legal action which he would quite easily win... he should go and see someone at the CAB office but this is quite clearly not permissable.
after he has been to the CAB, or even a solicitor (first half an hour is free in many places), he should write a letter to the boss (or get the solicitor to do it - though this would be for a small fee) refusing the reduction in pay and asking for a proper contract that they are obliged to provide him, this should not be a temporary contract as this is also illegal after certain lengths of service to protect workers rights.
good luck to your friend
Undercovers