It depends how it is done. For example, if your ex has been made a director and will be paid in dividends at the end of the financial year, then the income will be registered at that time.
Or he might have set up a limited company and they are placing work with him through that, he will have to do a company return in a year’s time and income will be recalculated then.
It’s unlikely they are just paying him cash because how would they account for it in their books? But if they are, yes that would be tax avoidance.
Try checking the companies house web site.