One has to ask how often he is flying? People have different ways of learning things and, especially with physical skills, different 'forget' rates.
Just an extreme for instance. We skied two weeks at Christmas, and sometimes three and then another week or ten days after Easter for over twenty years when we lived in Englnd. We were adequate skiers but it always took the first week before we got our skills back and even then when conditions got a bit rough, rocky or icy we'd go to pieces in a second. When we came to Canada we lived in Vancouver and after about five years got a cabin up here. The first Winter we skied most weekends and that was enough to imprint the skills for good. As we get older, and live in the resort now, we hardly go up at all, (strange that!) but first day up new season, no problem. Ice?, rocks? no problem. Don't do any exercise year to year, still ski relaxed and hardly get tired at all.
Maybe your friend might look at his flying pattern, maybe have a look at setting up lessons in a string that will imprint the skills better?