My husbands ex wife after 5 years of handing her cash for support her boyfriend is getting her to do child support through court, she wants money from him for back support when he has never been late but he missed a month because he lost his job. But he was out of work for 4 months but paid her with his 401k so what should we expect

If she never filed before, normally they do not give back support
( but some states do) and if you were paying her cash, sorry that was just stupid and most people know that you do checks and have proof of payment. If you can't prove you were paying them money, no cancelled checks, no receipts and they will not admit they got money, it never happened as for as the court is concerned.

To be honest it always should have been though the courts and now it will be done properly.

Also it will be set according to his wages, so if he is unemployed, the child support will be very very little each month.

He has receipts for everything until late oct she kept forgetting to give it to him my husband is very passive aggresssive.

Gather all your receipts for court. You may get stuck for any time you do not have proof of payment.