If he has a trainer, could he just do an occasional trainer workout to eliminate a lot of the external factors (wind, traffic, etc) that affect average speed in addition to fitness? Obviously not as good in absolute terms as a powermeter, but maybe an option to use what he might have on hand.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"