What can he do?

A friend of mine has 2 children and he gets paid every week and every week they take out 100. that's 200 for ea child it leves him with 110 a week . so thet's 440.00 a month wich he beleves the mother spends on her new boy friend. She also has a good job wich she may or may not have reported to childsuppert.
What can he do to reduce his payments