I would imagine your friend has a long term interest in flying a Part 21 aircraft. He can exercise the privileges of that licence using a Medical Declaration but, he cannot exercise the privileges on a Part 21 aircraft, except for the short term exemption contained in ORS 1471 which is to ease the problem of pilots regaining currency but is very limited.
There is another issue if you friend hasn't used his licence for a while, ICAO introduced a requirement for English Language Proficiency to be endorsed on all pilot licences back in 2008 and the CAA reissued all current pilot licences, if he was not current he may not have the ELP endorsement.
He does have the option of obtaining a Part FCL licence if he wants to operate Part 21 aircraft.