Travelling to Hong Kong and China during Covid

I have heard that non-Hong Kong/China residents/passengers will be denied entry into Hong Kong. Please could you advise on Hong Kong/China's latest rules and regulations in terms of Covid-19? If one cannot enter via Hong Kong, what about entering via Macao and/or mainland China?
I am enquiring on behalf of a Polish national who is flying to and arriving in Hong Kong on 8 June 2021. He will be staying in Hong Kong for 2 days only and fly to Cape Town, South Africa via Dubai.
He is a Polish national who will be vaccinated with the Johnson & Johnson Covid-19 vaccine on 22 May 2021 in Poland. He will depart Poland on 7 June 2021 and arrive in Hong Kong on 8 June 2021.
If this person has tested negatively for Covid-19 in Poland, 72 hours prior to arriving in Hong Kong on 8 June 2021, does he need to test again in Hong Kong, even though he will be staying for 2 days only?
If so, what is the cost of the test? Where will the test be done? At the Hong Kong airport?
Does he need to quarantine in Hong Kong and if so, how long is the quarantine period? He will be in Hong Kong for 2 days only. WOuld appreciate it if you could urgently assist. Many thanks, Brigitte