If he did find it in his garden that would quite impressive!
It seems to be a billon tetradrachm from Alexandria, Egypt (as above). It dates to 243/244 AD, and was issued for the Roman Emperor Gordian III.
EGYPT. Alexandria. Gordian III, 238-244. Tetradrachm RY 7 = 243/244. A K M ANT ΓOPΔΙANOC EY Laureate, draped and cuirassed bust of Gordian to right. Rev. L Z Eagle facing, with spread wings and its head turned to right, holding laurel wreath with its talons. Dattari (Savio) 4811 & 12422. Emmett 3402.7. Geissen 2671-2.
These coins were never intended to be anywhere near Britain. The handful that do appear are usually thought to be modern losses of ancient coins, but there are a few examples that might have drifted over in the purses of merchants etc.
96 tetradrachmae from the Greek world have somehow ended up in Britain and have been recorded on the PAS;
https://finds.org.uk/database/search/results/q/tetradrachm
Cool find!
(Image and description https://pro.coinarchives.com/a/lotviewer.php?LotID=1048344&AucID=2046&Lot=279)