Tax and Salary Questions (Hong Kong)

Hi I am a recruiter based in Grafton, MA USA. I work with global manufacturing clients to source and place professionals in Asia with engineering and manufacturing backgrounds. They have extended an offer of employment to my candidate however this is the first time they have hired a senior manager (he will be the General Manager for the plant) and they had the following questions:

The candidate studied abroad in the USA and whilst there applied and received a green card and USA passport. He has moved back to Hong Kong where he resides however he will be working and living during work hours in Dongguan. How do my client pay him and avoid too many taxes? They have a Hong Kong location as well. Should they pay him in monthly RMB and if so does Hong Kong apply any taxes as well as China to this income. If we paid through their Hong Kong location would he have to pay Hong Kong and Dongguan taxes on what he earns? Grateful for any answers