Wife demoted after return from maternity leave: ask the expert

My wife has recently returned to work after being off on maternity for 11 months. Whilst she was off, her job was advertised as a permanent post under a more junior title but with the same responsibilities. A person was hired but resigned before my wife returned. Upon returning, she agreed with her employer that she would work a four-day week and a new contract was issued. Like her previous contract, it stated that she would report direct to the managing director. However, she now finds herself in a position where all her business is now to be reported to another non executive director and that this director will sign off any commissions due. In all but title, she has been demoted. Upon returning she checked with the MD, finance and HR that her commission structure would remain the same as before maternity. All confirmed that it would. Since then she has overachieved against her targets, but the company have chosen not to pay her commission. Other employees have been paid their commission. Is this discrimination and what should she do?