If your friend has been fully vaccinated in Poland, he must have been vaccinated with a 2 dose vaccine (such as Moderna or Pfizer) or one dose of an approved single dose vaccine (such as Janssen) more than 14 days before arriving into the country..

He must have had both doses to be considered fully vaccinated. This applies in all cases, even if he has recently recovered from COVID-19 and has natural immunity. Each dose must be with the same (MHRA, EMA, Swissmedic or FDA) approved vaccine. For example, if his first dose was Moderna his second dose must also be Moderna.
He can show an EU Digital COVID Certificate (EU DCC), showing he's had a full course of an EMA or Swissmedic-approved vaccine in Poland. He can show either a digital or paper certificate.

Those who have had COVID-19 and have only had one dose of a 2 dose vaccine must follow the rules for unvaccinated arrivals. In which case he will be required upon arrival to :
quarantine on arrival at home or in the place he's staying
take a day 2 test after he arrives – on or before day 2.
Regardless of his vaccination status before he travels to England he must:
take a COVID-19 test 72 hours before departure at the most.
https://www.gov.uk/guidance/coronavirus ... to-england
book and pay for COVID-19 test – to be taken after arrival in England
https://www.gov.uk/find-travel-test-provider
Complete a passenger locator form
https://www.gov.uk/provide-journey-cont ... -travel-uk
Upon entry into England if he was fully vaccinated, he must take a day 2 test, on or before day 2. He will need to log the test online and will still receive the test result through email.

Wiec jezeli przylatujemy np. w piatek a wracamy w sobote wieczorem to test na dzien 2 trzeba kupic i zrobic chociazby zaraz przed wylotem.. jedynie co jest sprawdzane to na wylocie z Polski czy test zsotal zakupiony, w UK nikt nie sprawdzi nam podczas 2-3 dniowego pobytu czy ten test wykonalismy. No ale mimo wszystko, cos kupic trzeba bo moga na lot nie wpuscic.

If you have recently recovered from COVID but are no longer infectious, you should use a lateral flow device (LFD) test. LFD tests have a lower sensitivity than PCR or LAMP tests, so they are less likely to return a positive result from a historic infection. The LFD test must meet the minimum performance standards as set out above.