Your friend's husbands contract may say two things -
1. His continued employment shall be subject to a probationary period of service. If he proves unsatisfactory during that period, the employer should warn him and give him an opportunity to improve. However, the employee has a right to claim unfair dismissal, but only after a qualifying period of service of twelve months. Therefore after six months service, he has no rights to claim unfair dismissal.
2 The contract may say that he shall receive a payment by way of commission for bringing in business for the employer. That is a contract in which the employer may be in breach if he fails to show that the contractual payment was not due if the employee does not complete his probationary period of service. If the contract of employment says "If you sell our goods and services, we will pay you a commission" without any strings attached, they will be in breach of the contract if they don't pay.
If he has any documentation, he should take it to the CAB or to an employment rights lawyer. If he doesn't have any, he'll have an uphill struggle.